Oh, hai there!

I'm about to buy some of your physical coins as Christmas presents.

I understand that the cost of adding transactions is negligible.  Still, is there any enforcement mechanism?  Can I receive a handful of transactions and then just get to work on solving the block?

I seem to remember something in the protocol specs about checking the timestamp of the latest transaction in a solved block or something like that.  But it wasn't clear, and I can't remember where I saw it now.

Anyway, it's not hugely important, but it has been bugging me.