That's not a bug and was actually discussed on
gitter a few weeks ago.
To clarify, the documentation states that the return type is an id64 which only looks like a number, but is actually some ESO specific black magic to bring 64 bit integers to Lua:
Code:
* GetAchievementTimestamp(*integer* _achievementId_)
** _Returns:_ *id64* _timestamp_
It works differently and can only be used with certain functions that accept id64 as input, like Id64ToString:
Code:
* Id64ToString(*id64* _id_)
** _Returns:_ *string* _stringDesc_
The actual bug is that GetTimeStamp also states that it returns an id64, when in reality it returns a regular Lua number. Due to it working like that since launch and almost every addon relying on GetTimeStamp returning a 32bit integer, they won't change the type of that and will instead update the documentation to state the correct return type next update.