In the following context, which is later?
"and make payments retroactive to the original filing date (09/15/99) or to Sept. 25, 1985, whichever is later".
Is that exactly how it was written? Obviously, anything in 1999 is later than anything in 1985. It's a very strange thing to write, since it's clear what the later date is.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.