You have a Time value field on your form that will accept a range of time values i.e. 10:00 AM - 2:00 PM and you would like to set a field on your form with the total elapsed hours (4). The code here will calculate the total hours taking AM and PM into consideration.
Field name PartialDayTime is a Date/Time field that is set to accept only time values, allow multiple entries is checked and you are using the slider to enter values.
Field name Hours is a computed text field with the following in Default Value event
tmStart := @TextToTime(@Trim(@Left(@Text(PartialDayTime);"-")));
tmEnd := @TextToTime(@Trim(@Right(@Text(PartialDayTime);"-")));