If you get the message "ancester check-in ... is younger" then one of two
things is wrong:

  1.  The previous check-in was correct but your current clock is wrong.

  2.  The previous check-in contains a bad timestamp

If the problem is (1), then all you have to do is fix the clock on your
local machine.  If the problem is (2), then simply run "fossil ui", browse
to the previous check-in, click on the "Edit" link, and change the
check-in time;  (If multiple check-ins have been made using a bad clock,
you might want to go back and fix up several prior check-in times.)

I think this is a very sensible approach.  What would you prefer
Fossil do?  Check-in the change out-of-order and let the poor user
try to figure out what happened later?  (That seems scary to me.)
Or automatically adjust the timestamps?  (That seems even scarier.)
If you know of a better solution, please offer a suggestion.