Try any of strptime or convert command.
e.g.
| gentimes start=-1 | eval YourDate="3:21:34 AM 12/8/2014" | table YourDate
| eval epoch1=strptime(YourDate,"%H:%M:%S %p %m/%d/%Y")
| convert timeformat="%H:%M:%S %p %m/%d/%Y" mktime(YourDate) as epoch2