I'm gonna venture an educated guess - No, you cannot perform math on index time extractions.
However, you can math in a search. Once you get it the way you want it, you can create a macro so it can be called easily.
Have you read this doc? http://docs.splunk.com/Documentation/Splunk/6.0.1/Data/HowSplunkextractstimestamps
According to that doc, if you can configure props.conf to recognize the 'date field' as an event of the same sourcetype, and pull the date as a date time, then all subsequent events would default to that 'date time' because the subsequent events of that same sourcetype do not have a valid 'date time'.
It is tough without seeing the data or log file structure.