In the US, dates are quite important on all contracts.

I've been told that if I don't have clear dates, my contracts could get nullified in court.

Sounds like this is a difference between legal systems in Finland vs US. In Finland it's okay to sign a contract that remains in force indefinitely.

I'll bet there's ways to do that in the US, too, but I guess it needs some "secret sauce" to make it work.

Source: IANAL.

Good job! I think this could be a valuable service, but the differences between legal systems could be a blocker.