Creator : Calculate Hours/Minutes between two time fields

Creator : Calculate Hours/Minutes between two time fields

I have two time fields and need to know how many hours and minutes there are between them (and convert to decimal if possible). Every thing I've tried is either for datetime data types; won't let me do calculations on time fields or the function timeBetween() doesn't exist for creator.

This is further complicated by the start/end time data being held in a subform called Operating_Schedule.
  1. //Get basic info about the chosen location
  2. varLocationInfo = Locations[ID = input.Location];
  3. varStart = varLocationInfo.Operating_Schedule.Operating_Schedule_1_Opening_Hour;
  4. varClose = varLocationInfo.Operating_Schedule.Operating_Schedule_1_Closing_Time;
When run varStart returns 09:00:00 as time and varClose returns 17:30:00 as time

What I have tried :
  1. https://help.zoho.com/portal/en/community/topic/how-to-calculate-difference-between-time#2266000006605619
    This gave me the error Arithmatic functions not allowed on TIME values

  2. https://help.zoho.com/portal/en/community/topic/calculating-hours-between-2-time-fields-in-zoho-creator
    This is only for whole hours

  3. https://help.zoho.com/portal/en/community/topic/calculating-hours-between-2-time-fields-in-zoho-creator
    This just returns a null error as no matter what I do varStart.GetHour() returns null even though I have checked it is a time field and varStart contains 09:00:00 ... still returns null

  4. https://www.zoho.com/deluge/help/functions/datetime/time-between.html
    Frustratingly, doesn't work in Creator
Something this simple, shouldn't be this difficult - what am I missing ?