On the 31st the number of the next month is calculated incorrectly

I think this is also UTC / local time zone related

I don't think that its timezone problem. I change topic. Now in pager 3 dates. If today is March 31st, then 5th month displayed twice.

Could you reproduce this on a new Drupal installation, e.g. on simplytest.me ? Do you patch something in date_pager? Or Javascript changing content in frontend? What is the URL in the false 05 link?
I couldn't reproduce it with the steps given above on simplytest.me.

Can simplytest.me choose "current" date?
No, i don't patch date_pager.
Same trouble in Claro or Olivero theme. Screenshot:

Both 05 links linked to May i.e. /view-url?date=2025-05.
On March 31st it's also possible to come in to April i.e. to /view-url?date=2025-04, but this 04 link is missing in pager.

I could reproduce it locally. It was a problem on top of the timezone problem with the mapping of the current date to the dates in the pager. Please test and review 🐛 Paging seems to be done using UTC and not local time zone. Active