Need help understanding UR move-in timing and city office procedure

I need a bit of help with a move-in issue and want to check if I’m understanding things correctly.

I recently signed a new UR contract and I’m scheduled to collect the keys on December 5. My original plan was to move out of my current place and move into the new UR apartment within this month.

However, due to a family emergency, I have to go back to my country this sunday and won’t be able to come back until late December or early January. It’s impossible for me to physically move in before I leave.

My current landlord agreed to let me stay until end of Jan 25. So realistically, I’ll only be able to move into the UR apartment in early or mid-January.

My concern is the UR clause that says something like “you must move in within one month of signing / key pickup.” I’ve already signed and the contract starts on Dec 5, but I won’t be living there until January.

I’m also confused about what to do with the city office move-in/住民票 procedures. All of that was supposed to be done in December if I had moved as originally planned.

Does anyone know how strict UR actually is about the “move-in within one month” part? And is it okay to start contract in December but do the official city hall move-in notification in January (way after 14 days 😬)?

Any experiences or advice would help a lot.