We're talking about a database here, not the code that processes a few records at a time. That code should be future proofed. But while the database design should be future proof, the exact settings don't have to be. Keep things properly isolated and nobody will ever know how many bits the database takes to store a value, and you can change it behind their backs with a single ALTER.
And someone hasn't written code based and tested on the 32bit date that then starts blowing (or even worse subtly making mistakes) up when you switch to 64bits...
That code is going to work/break the same with or without the database being expanded. Only truly ridiculous code would depend on the fact that inserting a date in 2040 causes the database to reject that transaction. Anything else in the code involving dates should have unit tests but is entirely independent of the database.
