MessagePack can only have UTC in the timestamp. There is no specification for storing as local time. On the other hand, MessagePack for C# serializes DateTimeOffset, which is stored as local time.

example

2022-09-17T18:27:56.520+09:00 means local time + offset.

The MessagePack specification only accepts UTC for timestamp. Therefore, we expect it to be saved in the following format.

expect : ["2022-09-17T09:27:56.52Z", 540]
actual : ["2022-09-17T18:27:56.52Z", 540]