Lotus Notes: Difference between hour -

I have two editable fields (time 1 and time 2), both in the calendar / time type, 24hr format. Both are showing time for example, 20:15, and I have a calculation for the display field which will show the difference between the two fields.

I have tried the difference: = (time 1 - time 2),

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Formula in a math field:

  _diff: = time 1 - time 2; @ If (_IIFIF); 0; If the field type is the date time, you get a difference in hours / minutes if the field is of type number, you get a difference in the second.  
 Do not forget the @is error, otherwise your form will not be open until you have a valid default value for time 1 and time 2