Format datetime as UTC with strftime (for use in git)

I can format dates and time pretty flexibly with strftime, but from what I can see, this only will give times in my local timezone; I can add the timezone offset (e.g. '%Y %m %d-%H:%M:%S%z' for a particular time yields 2022 02 14 16:21:35-0800), but what I actually want is the datetime for UTC. (For that example, 2022 02 15 00:21:35.) Is this possible with strftime? (Context: I'm fiddling with git log --date=format trying to get a format which matches go.mod fake versions. The --date=format:'%foo%bar' option, for defining custom formats, delegates to system strftime. I'm on Mac OSX Monterey and using bash. EDIT: Solution was somewhat more complex due to being used in git; changed the question description to reflect that.)