I'm arriving at midnight, and have a jonesen for some chili crab (and pepper too), what time is the UDMC open till?

This website has a special "After Midnight" section

http://www.makantime.com

excerpt:
Macpherson/Serangoon - I can't believe nobody's noticed the Kopitiam on the corner where Macpherson meets Serangoon Rd, Open 24 hours, cheap, some of the best seafood and chilli crab in Singapore, no bus loads of Japanese tourists; friendly, bustling at all hours. In fact the Kopitiam chain has similar outlets all over the place. All 24hrs, all clean and cheap. go for it. There's nothing like a bungkus chilli (and a pepper) crab at 2.00am. - Craig McLeod


Some of the restaurants in the East Coast Seafood Centre (UMDC) close at 12 midnight.

http://www.jumboseafood.com.sg/branches.shtml

Perhaps some others might stay open later.

There's a place called the No Signboard in Gaylang. Think of a small 4 store strip mall with a small parking lot in the front. The parking lot is where they put the tables after the other shops close and one of the "stores" is the kitchen. Nothing but the locals go there and the chili crab is the best. As long as it doesn't rain your OK. Since it doesn't open up till 6 or so, my guess it is open late especially since the Singaporeans like to eat at all hours.