if system time changes what will dump do?
so it failed on system time change? because incremental backup depends on last full dump time!
Incrementals are indeed based on the system time (they save all inodes which have changed since the last full - or lower incremental - dump).
If the system time change in the future this should be rather harmless. But if the system time change in the past, crossing the time of a previous dump, the incremental dump could save more inodes than needed.
Besides, I wonder if there aren't some checks in the code (in either dump or restore) which would prevent such a case to work...