Improve and extend DateTime/DateTimeOffset generation

src/Hedgehog/Gen.fs

@@ -515,21 +515,32 @@ module Gen =
         return System.Guid bs
     }
 
-    /// Generates a random instant in time expressed as a date and time of day.
+    /// Generates a random DateTime using the specified range.
     [<CompiledName("DateTime")>]
-    let dateTime : Gen<System.DateTime> =
-        let minTicks =
-            System.DateTime.MinValue.Ticks
-        let maxTicks =
-            System.DateTime.MaxValue.Ticks
+    let dateTime (range : Range<DateTime>) : Gen<System.DateTime> =
         gen {
-            let! ticks =
-                Range.constantFrom
-                    (System.DateTime (2000, 1, 1)).Ticks minTicks maxTicks
-                |> integral
+            let! ticks = range |> Range.map (fun dt -> dt.Ticks) |> integral
             return System.DateTime ticks
         }
 
+    /// Generates a random DateTimeOffset using the specified range.
+    [<CompiledName("DateTimeOffset")>]
+    let dateTimeOffset (range : Range<DateTimeOffset>) : Gen<System.DateTimeOffset> =
+        gen {
+            let! ticks = range |> Range.map (fun dt -> dt.Ticks) |> integral
+            // Ensure there is no overflow near the edges when adding the offset
+            let minOffsetMinutes =
+              max 
+                (-14L * 60L)
+                ((DateTimeOffset.MaxValue.Ticks - ticks) / TimeSpan.TicksPerMinute * -1L)
+            let maxOffsetMinutes =
+              min 
+                (14L * 60L)
+                ((ticks - DateTimeOffset.MinValue.Ticks) / TimeSpan.TicksPerMinute)
+            let! offsetMinutes = int (Range.linearFrom 0 (Operators.int minOffsetMinutes) (Operators.int maxOffsetMinutes))
+            return System.DateTimeOffset(ticks, TimeSpan.FromMinutes (Operators.float offsetMinutes))
+        }
+
     //
     // Sampling
     //

tests/Hedgehog.Tests/GenTests.fs

@@ -13,7 +13,7 @@ open Xunit
 [<InlineData(256)>]
 [<InlineData(512)>]
 let ``dateTime creates System.DateTime instances`` count =
-    let actual = Gen.dateTime |> Gen.sample 0 count
+    let actual = Gen.dateTime (Range.constant System.DateTime.MinValue System.DateTime.MaxValue) |> Gen.sample 0 count
     actual
     |> List.distinct
     |> List.length
@@ -45,7 +45,7 @@ let ``dateTime randomly generates value between max and min ticks`` () =
         |> Random.run seed1 0
     let expected = System.DateTime ticks
 
-    let actual = Gen.dateTime
+    let actual = Gen.dateTime (Range.constant System.DateTime.MinValue System.DateTime.MaxValue)
 
     let result = actual |> Gen.toRandom |> Random.run seed0 0 |> Tree.outcome
     expected =! result
@@ -54,7 +54,9 @@ let ``dateTime randomly generates value between max and min ticks`` () =
 let ``dateTime shrinks to correct mid-value`` () =
     let result =
         property {
-            let! actual = Gen.dateTime
+            let! actual = 
+              Range.constantFrom (System.DateTime (2000, 1, 1)) System.DateTime.MinValue System.DateTime.MaxValue
+              |> Gen.dateTime
             System.DateTime.Now =! actual
         }
         |> Property.report