diff --git a/chai.js b/chai.js
index a2689f327..67b70508c 100644
--- a/chai.js
+++ b/chai.js
@@ -14,7 +14,7 @@ var used = [];
  * Chai version
  */
 
-exports.version = '4.0.2';
+exports.version = '4.1.0';
 
 /*!
  * Assertion Error
@@ -691,6 +691,16 @@ module.exports = function (chai, _) {
    *
    *     expect({a: 1, b: 2, c: 3}).to.include({a: 1, b: 2});
    *
+   * When the target is a Set or WeakSet, `.include` asserts that the given `val` is a
+   * member of the target. SameValueZero equality algorithm is used.
+   *
+   *     expect(new Set([1, 2])).to.include(2);
+   *
+   * When the target is a Map, `.include` asserts that the given `val` is one of
+   * the values of the target. SameValueZero equality algorithm is used.
+   *
+   *     expect(new Map([['a', 1], ['b', 2]])).to.include(2);
+   *
    * Because `.include` does different things based on the target's type, it's
    * important to check the target's type before using `.include`. See the `.a`
    * doc for info on testing a target's type.
@@ -699,8 +709,8 @@ module.exports = function (chai, _) {
    *
    * By default, strict (`===`) equality is used to compare array members and
    * object properties. Add `.deep` earlier in the chain to use deep equality
-   * instead. See the `deep-eql` project page for info on the deep equality
-   * algorithm: https://github.com/chaijs/deep-eql.
+   * instead (WeakSet targets are not supported). See the `deep-eql` project
+   * page for info on the deep equality algorithm: https://github.com/chaijs/deep-eql.
    *
    *     // Target array deeply (but not strictly) includes `{a: 1}`
    *     expect([{a: 1}]).to.deep.include({a: 1});
@@ -810,25 +820,24 @@ module.exports = function (chai, _) {
    * @api public
    */
 
-  function includeChainingBehavior () {
-    flag(this, 'contains', true);
+  function SameValueZero(a, b) {
+    return (_.isNaN(a) && _.isNaN(b)) || a === b;
   }
 
-  function isDeepIncluded (arr, val) {
-    return arr.some(function (arrVal) {
-      return _.eql(arrVal, val);
-    });
+  function includeChainingBehavior () {
+    flag(this, 'contains', true);
   }
 
   function include (val, msg) {
     if (msg) flag(this, 'message', msg);
 
-    _.expectTypes(this, ['array', 'object', 'string'], flag(this, 'ssfi'));
+    _.expectTypes(this, [
+      'array', 'object', 'string',
+      'map', 'set', 'weakset',
+    ]);
 
     var obj = flag(this, 'object')
-      , objType = _.type(obj).toLowerCase()
-      , isDeep = flag(this, 'deep')
-      , descriptor = isDeep ? 'deep ' : '';
+      , objType = _.type(obj).toLowerCase();
 
     // This block is for asserting a subset of properties in an object.
     if (objType === 'object') {
@@ -865,10 +874,62 @@ module.exports = function (chai, _) {
       return;
     }
 
-    // Assert inclusion in an array or substring in a string.
+    var isDeep = flag(this, 'deep')
+      , descriptor = isDeep ? 'deep ' : ''
+      , included = false;
+
+    switch (objType) {
+      case 'string':
+        included = obj.indexOf(val) !== -1;
+        break;
+
+      case 'weakset':
+        if (isDeep) {
+          var flagMsg = flag(this, 'message')
+            , ssfi = flag(this, 'ssfi');
+          flagMsg = flagMsg ? flagMsg + ': ' : '';
+
+          throw new AssertionError(
+            flagMsg + 'unable to use .deep.include with WeakSet',
+            undefined,
+            ssfi
+          );
+        }
+
+        included = obj.has(val);
+        break;
+
+      case 'map':
+        var isEql = isDeep ? _.eql : SameValueZero;
+        obj.forEach(function (item) {
+          included = included || isEql(item, val);
+        });
+        break;
+
+      case 'set':
+        if (isDeep) {
+          obj.forEach(function (item) {
+            included = included || _.eql(item, val);
+          });
+        } else {
+          included = obj.has(val);
+        }
+        break;
+
+      case 'array':
+        if (isDeep) {
+          included = obj.some(function (item) {
+            return _.eql(item, val);
+          })
+        } else {
+          included = obj.indexOf(val) !== -1;
+        }
+        break;
+    }
+
+    // Assert inclusion in collection or substring in a string.
     this.assert(
-        objType === 'string' || !isDeep ? ~obj.indexOf(val)
-                                        : isDeepIncluded(obj, val)
+      included
       , 'expected #{this} to ' + descriptor + 'include ' + _.inspect(val)
       , 'expected #{this} to not ' + descriptor + 'include ' + _.inspect(val));
   }
@@ -1385,7 +1446,7 @@ module.exports = function (chai, _) {
   /**
    * ### .above(n[, msg])
    *
-   * Asserts that the target is a number greater than the given number `n`.
+   * Asserts that the target is a number or a date greater than the given number or date `n` respectively.
    * However, it's often best to assert that the target is equal to its expected
    * value.
    *
@@ -1430,21 +1491,29 @@ module.exports = function (chai, _) {
     var obj = flag(this, 'object')
       , doLength = flag(this, 'doLength')
       , flagMsg = flag(this, 'message')
-      , ssfi = flag(this, 'ssfi');
+      , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
+      , ssfi = flag(this, 'ssfi')
+      , objType = _.type(obj).toLowerCase()
+      , nType = _.type(n).toLowerCase()
+      , shouldThrow = true;
 
     if (doLength) {
       new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
+    }
+    
+    if (!doLength && (objType === 'date' && nType !== 'date')) {
+      errorMessage = msgPrefix + 'the argument to above must be a date';
+    } else if (nType !== 'number' && (doLength || objType === 'number')) {
+      errorMessage = msgPrefix + 'the argument to above must be a number';
+    } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
+      var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
+      errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
     } else {
-      new Assertion(obj, flagMsg, ssfi, true).is.a('number');
+      shouldThrow = false;
     }
 
-    if (typeof n !== 'number') {
-      flagMsg = flagMsg ? flagMsg + ': ' : '';
-      throw new AssertionError(
-        flagMsg + 'the argument to above must be a number',
-        undefined,
-        ssfi
-      );
+    if (shouldThrow) {
+      throw new AssertionError(errorMessage, undefined, ssfi);
     }
 
     if (doLength) {
@@ -1459,8 +1528,9 @@ module.exports = function (chai, _) {
     } else {
       this.assert(
           obj > n
-        , 'expected #{this} to be above ' + n
-        , 'expected #{this} to be at most ' + n
+        , 'expected #{this} to be above #{exp}'
+        , 'expected #{this} to be at most #{exp}'
+        , n
       );
     }
   }
@@ -1472,8 +1542,8 @@ module.exports = function (chai, _) {
   /**
    * ### .least(n[, msg])
    *
-   * Asserts that the target is a number greater than or equal to the given
-   * number `n`. However, it's often best to assert that the target is equal to
+   * Asserts that the target is a number or a date greater than or equal to the given
+   * number or date `n` respectively. However, it's often best to assert that the target is equal to
    * its expected value.
    *
    *     expect(2).to.equal(2); // Recommended
@@ -1517,21 +1587,29 @@ module.exports = function (chai, _) {
     var obj = flag(this, 'object')
       , doLength = flag(this, 'doLength')
       , flagMsg = flag(this, 'message')
-      , ssfi = flag(this, 'ssfi');
+      , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
+      , ssfi = flag(this, 'ssfi')
+      , objType = _.type(obj).toLowerCase()
+      , nType = _.type(n).toLowerCase()
+      , shouldThrow = true;
 
     if (doLength) {
       new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
+    }
+
+    if (!doLength && (objType === 'date' && nType !== 'date')) {
+      errorMessage = msgPrefix + 'the argument to least must be a date';
+    } else if (nType !== 'number' && (doLength || objType === 'number')) {
+      errorMessage = msgPrefix + 'the argument to least must be a number';
+    } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
+      var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
+      errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
     } else {
-      new Assertion(obj, flagMsg, ssfi, true).is.a('number');
+      shouldThrow = false;
     }
 
-    if (typeof n !== 'number') {
-      flagMsg = flagMsg ? flagMsg + ': ' : '';
-      throw new AssertionError(
-        flagMsg + 'the argument to least must be a number',
-        undefined,
-        ssfi
-      );
+    if (shouldThrow) {
+      throw new AssertionError(errorMessage, undefined, ssfi);
     }
 
     if (doLength) {
@@ -1546,8 +1624,9 @@ module.exports = function (chai, _) {
     } else {
       this.assert(
           obj >= n
-        , 'expected #{this} to be at least ' + n
-        , 'expected #{this} to be below ' + n
+        , 'expected #{this} to be at least #{exp}'
+        , 'expected #{this} to be below #{exp}'
+        , n
       );
     }
   }
@@ -1558,7 +1637,7 @@ module.exports = function (chai, _) {
   /**
    * ### .below(n[, msg])
    *
-   * Asserts that the target is a number less than the given number `n`.
+   * Asserts that the target is a number or a date less than the given number or date `n` respectively.
    * However, it's often best to assert that the target is equal to its expected
    * value.
    *
@@ -1603,21 +1682,29 @@ module.exports = function (chai, _) {
     var obj = flag(this, 'object')
       , doLength = flag(this, 'doLength')
       , flagMsg = flag(this, 'message')
-      , ssfi = flag(this, 'ssfi');
+      , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
+      , ssfi = flag(this, 'ssfi')
+      , objType = _.type(obj).toLowerCase()
+      , nType = _.type(n).toLowerCase()
+      , shouldThrow = true;
 
     if (doLength) {
       new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
+    }
+
+    if (!doLength && (objType === 'date' && nType !== 'date')) {
+      errorMessage = msgPrefix + 'the argument to below must be a date';
+    } else if (nType !== 'number' && (doLength || objType === 'number')) {
+      errorMessage = msgPrefix + 'the argument to below must be a number';
+    } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
+      var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
+      errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
     } else {
-      new Assertion(obj, flagMsg, ssfi, true).is.a('number');
+      shouldThrow = false;
     }
 
-    if (typeof n !== 'number') {
-      flagMsg = flagMsg ? flagMsg + ': ' : '';
-      throw new AssertionError(
-        flagMsg + 'the argument to below must be a number',
-        undefined,
-        ssfi
-      );
+    if (shouldThrow) {
+      throw new AssertionError(errorMessage, undefined, ssfi);
     }
 
     if (doLength) {
@@ -1632,8 +1719,9 @@ module.exports = function (chai, _) {
     } else {
       this.assert(
           obj < n
-        , 'expected #{this} to be below ' + n
-        , 'expected #{this} to be at least ' + n
+        , 'expected #{this} to be below #{exp}'
+        , 'expected #{this} to be at least #{exp}'
+        , n
       );
     }
   }
@@ -1645,8 +1733,8 @@ module.exports = function (chai, _) {
   /**
    * ### .most(n[, msg])
    *
-   * Asserts that the target is a number less than or equal to the given number
-   * `n`. However, it's often best to assert that the target is equal to its
+   * Asserts that the target is a number or a date less than or equal to the given number
+   * or date `n` respectively. However, it's often best to assert that the target is equal to its
    * expected value.
    *
    *     expect(1).to.equal(1); // Recommended
@@ -1689,21 +1777,29 @@ module.exports = function (chai, _) {
     var obj = flag(this, 'object')
       , doLength = flag(this, 'doLength')
       , flagMsg = flag(this, 'message')
-      , ssfi = flag(this, 'ssfi');
+      , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
+      , ssfi = flag(this, 'ssfi')
+      , objType = _.type(obj).toLowerCase()
+      , nType = _.type(n).toLowerCase()
+      , shouldThrow = true;
 
     if (doLength) {
       new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
+    }
+    
+    if (!doLength && (objType === 'date' && nType !== 'date')) {
+      errorMessage = msgPrefix + 'the argument to most must be a date';
+    } else if (nType !== 'number' && (doLength || objType === 'number')) {
+      errorMessage = msgPrefix + 'the argument to most must be a number';
+    } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
+      var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
+      errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
     } else {
-      new Assertion(obj, flagMsg, ssfi, true).is.a('number');
+      shouldThrow = false;
     }
 
-    if (typeof n !== 'number') {
-      flagMsg = flagMsg ? flagMsg + ': ' : '';
-      throw new AssertionError(
-        flagMsg + 'the argument to most must be a number',
-        undefined,
-        ssfi
-      );
+    if (shouldThrow) {
+      throw new AssertionError(errorMessage, undefined, ssfi);
     }
 
     if (doLength) {
@@ -1718,8 +1814,9 @@ module.exports = function (chai, _) {
     } else {
       this.assert(
           obj <= n
-        , 'expected #{this} to be at most ' + n
-        , 'expected #{this} to be above ' + n
+        , 'expected #{this} to be at most #{exp}'
+        , 'expected #{this} to be above #{exp}'
+        , n
       );
     }
   }
@@ -1730,8 +1827,8 @@ module.exports = function (chai, _) {
   /**
    * ### .within(start, finish[, msg])
    *
-   * Asserts that the target is a number greater than or equal to the given
-   * number `start`, and less than or equal to the given number `finish`.
+   * Asserts that the target is a number or a date greater than or equal to the given
+   * number or date `start`, and less than or equal to the given number or date `finish` respectively.
    * However, it's often best to assert that the target is equal to its expected
    * value.
    *
@@ -1773,24 +1870,35 @@ module.exports = function (chai, _) {
   Assertion.addMethod('within', function (start, finish, msg) {
     if (msg) flag(this, 'message', msg);
     var obj = flag(this, 'object')
-      , range  = start + '..' + finish
       , doLength = flag(this, 'doLength')
       , flagMsg = flag(this, 'message')
-      , ssfi = flag(this, 'ssfi');
+      , msgPrefix = ((flagMsg) ? flagMsg + ': ' : '')
+      , ssfi = flag(this, 'ssfi')
+      , objType = _.type(obj).toLowerCase()
+      , startType = _.type(start).toLowerCase()
+      , finishType = _.type(finish).toLowerCase()
+      , shouldThrow = true
+      , range = (startType === 'date' && finishType === 'date')
+          ? start.toUTCString() + '..' + finish.toUTCString()
+          : start + '..' + finish;
 
     if (doLength) {
       new Assertion(obj, flagMsg, ssfi, true).to.have.property('length');
+    }
+
+    if (!doLength && (objType === 'date' && (startType !== 'date' || finishType !== 'date'))) {
+      errorMessage = msgPrefix + 'the arguments to within must be dates';
+    } else if ((startType !== 'number' || finishType !== 'number') && (doLength || objType === 'number')) {
+      errorMessage = msgPrefix + 'the arguments to within must be numbers';
+    } else if (!doLength && (objType !== 'date' && objType !== 'number')) {
+      var printObj = (objType === 'string') ? "'" + obj + "'" : obj;
+      errorMessage = msgPrefix + 'expected ' + printObj + ' to be a number or a date';
     } else {
-      new Assertion(obj, flagMsg, ssfi, true).is.a('number');
+      shouldThrow = false;
     }
 
-    if (typeof start !== 'number' || typeof finish !== 'number') {
-      flagMsg = flagMsg ? flagMsg + ': ' : '';
-      throw new AssertionError(
-        flagMsg + 'the arguments to within must be numbers',
-        undefined,
-        ssfi
-      );
+    if (shouldThrow) {
+      throw new AssertionError(errorMessage, undefined, ssfi);
     }
 
     if (doLength) {
@@ -2005,6 +2113,7 @@ module.exports = function (chai, _) {
     var isNested = flag(this, 'nested')
       , isOwn = flag(this, 'own')
       , flagMsg = flag(this, 'message')
+      , obj = flag(this, 'object')
       , ssfi = flag(this, 'ssfi');
 
     if (isNested && isOwn) {
@@ -2016,9 +2125,17 @@ module.exports = function (chai, _) {
       );
     }
 
+    if (obj === null || obj === undefined) {
+      flagMsg = flagMsg ? flagMsg + ': ' : '';
+      throw new AssertionError(
+        flagMsg + 'Target cannot be null or undefined.',
+        undefined,
+        ssfi
+      );
+    }
+
     var isDeep = flag(this, 'deep')
       , negate = flag(this, 'negate')
-      , obj = flag(this, 'object')
       , pathInfo = isNested ? _.getPathInfo(obj, name) : null
       , value = isNested ? pathInfo.value : obj[name];
 
@@ -7720,8 +7837,6 @@ module.exports = function compareByInspect(a, b) {
  *
  * @param {Mixed} obj constructed Assertion
  * @param {Array} type A list of allowed types for this assertion
- * @param {Function} ssfi starting point for removing implementation frames from
- *                        stack trace of AssertionError
  * @namespace Utils
  * @name expectTypes
  * @api public
@@ -7731,8 +7846,9 @@ var AssertionError = require('assertion-error');
 var flag = require('./flag');
 var type = require('type-detect');
 
-module.exports = function expectTypes(obj, types, ssfi) {
+module.exports = function expectTypes(obj, types) {
   var flagMsg = flag(obj, 'message');
+  var ssfi = flag(obj, 'ssfi');
 
   flagMsg = flagMsg ? flagMsg + ': ' : '';
 
@@ -7740,7 +7856,7 @@ module.exports = function expectTypes(obj, types, ssfi) {
   types = types.map(function (t) { return t.toLowerCase(); });
   types.sort();
 
-  // Transforms ['lorem', 'ipsum'] into 'a lirum, or an ipsum'
+  // Transforms ['lorem', 'ipsum'] into 'a lorem, or an ipsum'
   var str = types.map(function (t, index) {
     var art = ~[ 'a', 'e', 'i', 'o', 'u' ].indexOf(t.charAt(0)) ? 'an' : 'a';
     var or = types.length > 1 && index === types.length - 1 ? 'or ' : '';
@@ -8257,7 +8373,7 @@ function formatValue(ctx, value, recurseTimes) {
           var container = document.createElementNS(ns, '_');
 
           container.appendChild(value.cloneNode(false));
-          html = container.innerHTML
+          var html = container.innerHTML
             .replace('><', '>' + value.innerHTML + '<');
           container.innerHTML = '';
           return html;
diff --git a/lib/chai.js b/lib/chai.js
index 50de77193..c1afbee92 100644
--- a/lib/chai.js
+++ b/lib/chai.js
@@ -10,7 +10,7 @@ var used = [];
  * Chai version
  */
 
-exports.version = '4.0.2';
+exports.version = '4.1.0';
 
 /*!
  * Assertion Error
diff --git a/package.json b/package.json
index 5e65d6eef..d0c86fc69 100644
--- a/package.json
+++ b/package.json
@@ -17,7 +17,7 @@
     "Veselin Todorov <hi@vesln.com>",
     "John Firebaugh <john.firebaugh@gmail.com>"
   ],
-  "version": "4.0.2",
+  "version": "4.1.0",
   "repository": {
     "type": "git",
     "url": "https://github.com/chaijs/chai"