Reverting -current by date.

>From time to time it would be handy to revert freebsd-current to
an older, well-behaved revision. 

Is there a mechanism for identifying revision numbers that
will at least compile and boot, by date? 

In my case buildworld seems to be markedly slower than, say,
six months ago. Maybe it's hardware, maybe something else. Is
there a way to pick a revision number to revert to, that's
better than merely guessing? 

Thanks for reading,

bob prohaska