How to calculate time taken?

I am working on a form for capturing time taken cooking particular food item by the cook of the house. For example i have question asking start and end of the cooking. How can i calculate or get the time taken to cook the food ? i have following data

1b) What time did you start cooking ${A_Dish}? 1c) What time did you finish cooking ${A_Dish}?
06:25:00.000+05:45 06:40:00.000+05:45

Thank you.

@rshrestha_winrock, we have a bunch of workarounds discussed previously (it should help you meet your needs):

Another one …

One more …

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