Add TimeSettings (Start, End and TTL) (#661)

* time

* .

* UT

* using JetBrains.Annotations;

src/WireMock.Net/Extensions/TimeSettingsExtensions.cs

@@ -0,0 +1,35 @@
+using System;
+using JetBrains.Annotations;
+using WireMock.Models;
+
+namespace WireMock.Extensions
+{
+    internal static class TimeSettingsExtensions
+    {
+        public static bool IsValid([CanBeNull] this ITimeSettings settings)
+        {
+            if (settings == null)
+            {
+                return true;
+            }
+
+            var now = DateTime.Now;
+            var start = settings.Start != null ? settings.Start.Value : now;
+            DateTime end;
+            if (settings.End != null)
+            {
+                end = settings.End.Value;
+            }
+            else if (settings.TTL != null)
+            {
+                end = start.AddSeconds(settings.TTL.Value);
+            }
+            else
+            {
+                end = DateTime.MaxValue;
+            }
+
+            return now >= start && now <= end;
+        }
+    }
+}