A few datetime fixes

src/Discord.Net.Core/Utils/DateTimeUtils.cs

@@ -13,7 +13,7 @@ namespace Discord
         public static DateTimeOffset FromSnowflake(ulong value)
             => FromUnixMilliseconds((long)((value >> 22) + 1420070400000UL));
         public static ulong ToSnowflake(DateTimeOffset value)
-            => (ulong)(ToUnixMilliseconds(value) - 1420070400000L) << 22;
+            => ((ulong)ToUnixMilliseconds(value) - 1420070400000UL) << 22;
 
         public static DateTimeOffset FromTicks(long ticks)
             => new DateTimeOffset(ticks, TimeSpan.Zero);

src/Discord.Net.Core/Utils/Preconditions.cs

@@ -185,9 +185,12 @@ namespace Discord
         // Bulk Delete
         public static void YoungerThanTwoWeeks(ulong[] collection, string name)
         {
-            var minimum = DateTimeUtils.ToSnowflake(DateTimeOffset.Now.Subtract(TimeSpan.FromMilliseconds(1209540000)));
+            var minimum = DateTimeUtils.ToSnowflake(DateTimeOffset.UtcNow.Subtract(TimeSpan.FromDays(14)));
             for (var i = 0; i < collection.Length; i++)
-                if (collection[i] <= minimum) throw new ArgumentOutOfRangeException(name, "Messages must be younger than two weeks to delete.");
+            {
+                if (collection[i] <= minimum) 
+                    throw new ArgumentOutOfRangeException(name, "Messages must be younger than two weeks old.");
+            }
         }
     }
 }