Why don't they collide at the source level? I'm assuming the db updates get checked into your central source code repo. Wouldn't the collision happen at that time?

What happens when developers create branches and use time stamps? Developer A working on trunk might check in db updates at 4pm and 5pm. Developer B working on a branch checks db updates into his branch at 4:30pm and 5:30pm. When working on the branch his db updates don't affect the "trunk" db. As soon as he merges does his 4:30pm update get run on the "trunk" db, or is it skipped over because the 5pm db update already ran, and only the 5:30pm change gets run?

Where I come from we use int values for our db update script names. When I merge from a branch I renumber my db update script file names starting at the tip int value +1 of the trunk db update.

fyi - I'm not coming from a Ruby/Rails perspective, if that matters.