Check that messages older than two weeks are not passed to bulk delete

This resolves #477

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

@@ -12,6 +12,8 @@ namespace Discord
 
         public static DateTimeOffset FromSnowflake(ulong value)
             => FromUnixMilliseconds((long)((value >> 22) + 1420070400000UL));
+        public static ulong ToSnowflake(DateTimeOffset value)
+            => (ulong)(ToUnixMilliseconds(value) - 1420070400000L) << 22;
 
         public static DateTimeOffset FromTicks(long ticks)
             => new DateTimeOffset(ticks, TimeSpan.Zero);

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

@@ -181,5 +181,13 @@ namespace Discord
             if (msg == null) return new ArgumentException($"Value must be less than {value}", name);
             else return new ArgumentException(msg, name);
         }
+
+        // Bulk Delete
+        public static void NoMessageOlderThanTwoWeeks(ulong[] collection, string name)
+        {
+            var minimum = DateTimeUtils.ToSnowflake(DateTimeOffset.Now.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.");
+        }
     }
 }

src/Discord.Net.Rest/DiscordRestApiClient.cs

@@ -484,6 +484,7 @@ namespace Discord.API
             Preconditions.NotNull(args, nameof(args));
             Preconditions.NotNull(args.MessageIds, nameof(args.MessageIds));
             Preconditions.AtMost(args.MessageIds.Length, 100, nameof(args.MessageIds.Length));
+            Preconditions.NoMessageOlderThanTwoWeeks(args.MessageIds, nameof(args.MessageIds));
             options = RequestOptions.CreateOrClone(options);
 
             switch (args.MessageIds.Length)