Construct a formula to track time spent in business hours only

I am in need of some assistance. I need to form a calculation for my employer. The Help Desk is opened from 7:30am to 6:30pm. I am in charge of excel reports for email support to ensure we are meeting all SLAs. If an email comes in at 8:00am and we have not responded to this user until the next day at 9:15am, we need to track the actual time it took to respond. How can we construct a formula to take into consideration the 11 hour work day from 7:30am to 6:30pm.

This is what i have so far. Again, it does not take into consideration our business hours and actual time it took to respond.

Please help.

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This formula will calculate business hours assuming that the start and end times are always within the business hours

=(NETWORKDAYS(A2,B2)-1)*("7:30"-"18:30")+MOD(B2,1)-MOD(A2,1)

where A2 has start time/date and B2 end time/date

format result cell as [h]:mm

If the email might come in outside of work hours then change to this version

=(NETWORKDAYS(A2,B2)-1)*("7:30"-"18:30")+MOD(B2,1)-MEDIAN(NETWORKDAYS(A2,A2)*MOD(A2,1),"7:30","18:30")

Can you give me the details for a calculation that doesn't give what you want - what dates/times do you have, what result do you expect?