It's a standard sealed lead acid battery:
https://www.rapidonline.com/yuasa-yuvolt-yucel-y1-2-6-valve-...
https://static.rapidonline.com/pdf/18-5207_v2.pdf
Design spec is up to 5 years. The one in the photo looks like it may have a date written in Sharpie on the side. I don't think it's 25 years old though. The "090323" on top could be DDMMYY indicating a battery from 2023.
I agree with the 2023 decode there, but there is still a bit of a puzzle:
If someone knew to replace the battery just three years ago, I would expect they also would have enough knowledge to set the clock itself. Or at least explain what was going on. Maybe this person isn't connected with the building anymore. But I would expect the new owners would be able to contact the old owners for help on this instead of sending a generalized call for help.
I would GUESS that someone tried figuring it out in 2023 and replaced the battery as troubleshooting. If it were me, I'd probably do the same then stop when I hit a wall so I didn't damage it further. I don't wanna be the guy who damages a 100+ year old public clock
I could just hear the clockmaker James Martin calling me a butcher in his disapproving manner
Could also be YYMMDD In most of Europe it would be DDMMYY - but seeing how the English still measure stuff in feet and inches I wouldn’t be surprised if they have some illogical way of structuring the dates as well
YYMMDD is standard(ish) ISO 8601 - the new versions of the spec require a 4 digit year but older ones allowed it, and ISO 8601 is also commonly used in manufacturing.
YMD is Sweden or Hungary. MDY is US and pre-1960s UK. YDM is Latvia. DYM and MYD are up for grabs.