How to calculate duration/set-up duration formula in Custom records
If you need to define a field for calculating duration - for example, to calculate the duration of a room booking in a Custom record designed to track room rental details - do the following:
1) Create a Count field for the Start Time (e.g. Count1), and put a check mark next to "Display as Time of Day."
2) Create another Count field for the End time (e.g. Count2), and put a check mark next to “Display as Time of Day."
3) Create a third Count field for the duration (e.g. Count3). For this field, put a check mark next to “Display as Duration." When defining this field, edit the "Formula or Checkbox Choices" field to enter the formula for the duration Count field is as follows: