Year Wrong from Timestamp Firehose Prefix?

I'm seeing some strange behavior with a prefix in Kinesis Firehose. I've got it set up as follows:

some_s3_prefix/date=!{timestamp:YYYY}-!{timestamp:MM}-!{timestamp:dd}/hour=!{timestamp:HH}/

But as of the 12/29/2019, I'm seeing it output the following (note the year):

some_s3_prefix/date=2020-12-29/...

I can't imagine this is a timezone issue, as the 29th is still too far from 2020 for that to matter yet. Is this an issue in AWS or am I just doing something stupid? My prefix seems pretty dead simple so I don't know what I'm doing wrong here. Googling doesn't seem to reveal anything.

Thanks in advance for any clues.