Example data

id TimeElapsed User
1 00:01:44 Bob
2 00:01:46 Bob

To sum a date time field first convert the field data to double with an expression using the CDbl Function.

exp1: CDbl([TimeElapsed])

Set the total type to Sum.

Then create another expression to convert the data from the first expression back to a date time field using the CVDate function.

exp2: CVDate([exp1])

Set the total value to Expression.

TimeElapsed User
00:03:30 Bob