The time meta format is TimeT so your declaration should be correct. Depending on the parser logic, you may try to only register the time meta without running the getTime function and see if the time meta is registered correctly. You can enable detailed stats from Decoder Explore -> parsers -> config -> detailed.stats=yes and this will show all the metas each parser registered in the stats. This change should be done for debugging only because it is expensive in processing.
As far as I know the bad cast error is happening because internally TimeT is a 64 bit integer while the lua engine currently in use only supports 32 bit integer, hence the error. My understaning (but I'd be happy to be wrong) is that until the lua engine will be upgraded, you cannot really deal with TimeT meta keys.