Simple method to find the number of seconds between two (non-standard) date formats?

Not sure what you mean by "without calculation". The following does the calculation...

date1="Thu Mar 5 09:15:27 2015"
date2="Thu Mar 5 09:30:58 2015"

printf "%s\n" $(( $(date -d "$date2" "+%s") - $(date -d "$date1" "+%s") ))