Question about timezones in commit & tag dates

Hi, while working on correct key rollover and verifying signatures for past commits for the ssh signing topic i am trying to understand how Git deals with timestamps for commits & tags. For ssh signing the user will manage expiration times within their allowedSigners file. Those timestamps do not carry a timezone and i would assume a user (or automatic generation of this file) will assume the systems timezone for them. Therefore i wanted to pass the commit & tags timestamps adjusted to the system timezone to make sure key rollover will have no gaps or failed verification's especially when commit and system timezone differ greatly and might roll over to another day.

However the commit & tags structs only seem to carry the objects timestamp as is, simply ignoring any timezone information. For the ssh feature i can easily enough parse the ident line again from the object header. But while looking at the usage of the existing date fields i can see that objects are sometimes sorted on and compared by these dates. When commands provide cut off times (--since) i think they might include or exclude commits erroneously when they were made in a different timezone around the cutoff date. ("log --since" indeed gives me some unexpected results when mixing multiple timezones. Based on some simple testing i think it just stops output when a commit falls outside of this window, even though there might be one before it wich is within)

Is my understanding of this correct and this the expected behaviour? I think generally for git this does not matter much. But in certain situations this is problematic.

I would have assumed that git would either add the timezone as well or adjust the commit timestamp upon populating the date field in the commit struct to UTC but i could not find anything like it.

Best regards, Fabian