Add support for getting the dominical letter for a year

build/ical.js

@@ -4821,6 +4821,17 @@ ICAL.TimezoneService = (function() {
       return this.dayOfYear() - delta;
     },
 
+    /**
+     * Get the dominical letter for the current year. Letters range from A - G
+     * for common years, and AG to GF for leap years.
+     *
+     * @param {Number} yr           The year to retrieve the letter for
+     * @return {String}             The dominical letter.
+     */
+    getDominicalLetter: function() {
+      return ICAL.Time.getDominicalLetter(this.year);
+    },
+
     /**
      * Finds the nthWeekDay relative to the current month (not day).  The
      * returned value is a day relative the month that this month belongs to so
@@ -5629,6 +5640,24 @@ ICAL.TimezoneService = (function() {
     return t;
   };
 
+  /**
+   * Get the dominical letter for the given year. Letters range from A - G for
+   * common years, and AG to GF for leap years.
+   *
+   * @param {Number} yr           The year to retrieve the letter for
+   * @return {String}             The dominical letter.
+   */
+  ICAL.Time.getDominicalLetter = function(yr) {
+    var LTRS = "GFEDCBA";
+    var dom = (yr + (yr / 4 | 0) + (yr / 400 | 0) - (yr / 100 | 0) - 1) % 7;
+    var isLeap = ICAL.Time.isLeapYear(yr);
+    if (isLeap) {
+      return LTRS[(dom + 6) % 7] + LTRS[dom];
+    } else {
+      return LTRS[dom];
+    }
+  };
+
   /**
    * January 1st, 1970 as an ICAL.Time.
    * @type {ICAL.Time}

lib/ical/time.js

@@ -387,6 +387,17 @@
       return this.dayOfYear() - delta;
     },
 
+    /**
+     * Get the dominical letter for the current year. Letters range from A - G
+     * for common years, and AG to GF for leap years.
+     *
+     * @param {Number} yr           The year to retrieve the letter for
+     * @return {String}             The dominical letter.
+     */
+    getDominicalLetter: function() {
+      return ICAL.Time.getDominicalLetter(this.year);
+    },
+
     /**
      * Finds the nthWeekDay relative to the current month (not day).  The
      * returned value is a day relative the month that this month belongs to so
@@ -1195,6 +1206,24 @@
     return t;
   };
 
+  /**
+   * Get the dominical letter for the given year. Letters range from A - G for
+   * common years, and AG to GF for leap years.
+   *
+   * @param {Number} yr           The year to retrieve the letter for
+   * @return {String}             The dominical letter.
+   */
+  ICAL.Time.getDominicalLetter = function(yr) {
+    var LTRS = "GFEDCBA";
+    var dom = (yr + (yr / 4 | 0) + (yr / 400 | 0) - (yr / 100 | 0) - 1) % 7;
+    var isLeap = ICAL.Time.isLeapYear(yr);
+    if (isLeap) {
+      return LTRS[(dom + 6) % 7] + LTRS[dom];
+    } else {
+      return LTRS[dom];
+    }
+  };
+
   /**
    * January 1st, 1970 as an ICAL.Time.
    * @type {ICAL.Time}

test/time_test.js

@@ -494,6 +494,48 @@ suite('icaltime', function() {
 
   });
 
+  suite('#getDominicalLetter', function() {
+    test('instance', function() {
+      var subject = function(yr) {
+        return (new ICAL.Time({ year: yr })).getDominicalLetter();
+      };
+      assert.equal(subject(1989), "A");
+      assert.equal(subject(1990), "G");
+      assert.equal(subject(1991), "F");
+      assert.equal(subject(1993), "C");
+      assert.equal(subject(1994), "B");
+      assert.equal(subject(1997), "E");
+      assert.equal(subject(1998), "D");
+
+      assert.equal(subject(2000), "BA");
+      assert.equal(subject(2004), "DC");
+      assert.equal(subject(2008), "FE");
+      assert.equal(subject(2012), "AG");
+      assert.equal(subject(2016), "CB");
+      assert.equal(subject(2020), "ED");
+      assert.equal(subject(2024), "GF");
+
+    });
+    test('static', function() {
+      var subject = ICAL.Time.getDominicalLetter;
+      assert.equal(subject(1989), "A");
+      assert.equal(subject(1990), "G");
+      assert.equal(subject(1991), "F");
+      assert.equal(subject(1993), "C");
+      assert.equal(subject(1994), "B");
+      assert.equal(subject(1997), "E");
+      assert.equal(subject(1998), "D");
+
+      assert.equal(subject(2000), "BA");
+      assert.equal(subject(2004), "DC");
+      assert.equal(subject(2008), "FE");
+      assert.equal(subject(2012), "AG");
+      assert.equal(subject(2016), "CB");
+      assert.equal(subject(2020), "ED");
+      assert.equal(subject(2024), "GF");
+    });
+  });
+
   suite('#nthWeekDay', function() {
     suite('negative', function() {
       test('last saturday in Sept 2012 (before current day)', function() {