Here is a list of the variables that you can include in the subject line (you will have to scroll down a bit):
Also, try these: $job.earliestTime$
and $job.latestTime$
There is no straightforward way I guess. Have a look at the similar post.
http://answers.splunk.com/answers/63372/how-to-include-searched-date-or-time-range-in-alert-report