How to calculate elapsed time between to days excluding weekends.

The number format dd:hh:mm doesn't work for durations over 31:59:59 because the dd part is actually a day of the month so 31 is actually 31st January, go to 32 days and it reverts to 1 (1st Feb).

You either have to use decimals, i.e. just format as a number with 2 decimal places so you'd get 83.68 for the above example (0.68 of a day representing 16:23)....or if you want to see 83:16:23 then try this formula

=NETWORKDAYS(C10,D10)-1-(MOD(C10,1)>MOD(D10,1))&":"&TEXT(D10-C10,"hh:mm")

Note: this gives a text result so it's not so easy to use in any further calculations you may require