Datepicker Timespan

Running into a road block. I have two datepickers showing time in 12 hour format. I am trying to find the TimeSpan between the two. I have tried several iterations of Timespan
TimeSpan diff = ${endtime}-${starttime};
TimeSpan diff = ${endtime.Subtract(${starttime})};

Nothing seems to work...can someone tell me what I am missing.
endtime is set as DateTime?
starttime is set as DateTime?
they are both set properties from a SQL database
bdc.Time_In (datetime)
bdc_Time_Out (datetime)

Thanks in advance for any help

You might need to check this thread: