What is the result you are expecting? In the first line, would you expect the EventStartTime to be 11am and end time to be 1pm? Also, are you doing this in the script?
You can use the maketime(hr,min,s) function to convert the start and end offsets into minutes, and add them to the shift start.
eventstarttime=SHIFTSTARTTIME + maketime(0,StartOffset,0)
This may format to a float (decimal), if this happens use the time() function to correct it back to a
eventstarttime=time( SHIFTSTARTTIME + maketime(0,StartOffset,0) )
Let me know if this is what you need.