When in processing time mode:

• Add debug assert in SourceOperator(s) to validate timestamps are monotonically increasing
• After a checkpoint restore, if due to clock skew the new process Date_t::now() sees timestamps behind the last watermark, sleep until the wallclock time has made it to the watermark
• Add our own wrapper around curTimeMillis64() function that guarantees monotonically increasing timestamps. This wrapper can look something like this:

uint64_t curTimeMillis64_monotonic() 
{
   static uint64_t val = 0;      <== TODO make this robust for multiple SPs in the same process
   uint64_t newVal = curTimeMillis64(); 
   if (newVal > val) { 
     val = newVal;
   } 
  return val; 
}