A medical examiner who performed the autopsy on a slain Staten Island fire marshal admitted the victim could have been killed a full day earlier than prosecutors have claimed.

The testimony of Dr. Kristin Roman backed defense lawyers' arguments that Janet Mercereau -- on trial for allegedly killing her husband, Doug -- could have been out running errands with her kids at the time.

Prosecutors have accused Mercereau, 39, of shooting her husband three times in the head at around 6:30 a.m. on Dec. 2, 2007, as her kids were sleeping.

Roman testified that the 39-year-old victim had to have been killed between three and 30 hours earlier.