Date Arithmetic in Unix Scripting

Unix has date function which is good to add or subtract days / time from current date but has lots of limitation. I found no easy way to do serious date-time arithmetic using it. One possible way is to convert date-time to seconds (since epoch) and then manipulate it.

#Assign a date to variable. 
startDateTime="2013-04-28 00:00:00"

#Converts date to epoch and assign it to variable. 
#One should be careful as it may accept it as date in only few predefined formats
startDateInSecs=`date -d "$StartDateTime" +"%s"`

#Perform manipulation by adding or subtracting time. 
#For example to add 2 hours
let newDateInSecs=$((StartDateInSecs + 2*3600))

#Then print or assign the date to a variable in required format.
NewDateTime=`date -d @"$newDateInSecs" +"%Y-%m-%d %H:%M:%S"`
echo NewDateTime

#Do note @ before variable $newDateInSecs. That shows date-time is in seconds since epoch