SLA calculation

As this is posted in the formulas forum, here is a purely formula based proposal.
The duration left for any particular day is displayed using the MIN function and is similar to: =MIN(TIME(20,0,0)-TIME(8,0,0),B3-C3)
The end time for any particular day is displayed using formulas similar to: =IF(E3>0,WORKDAY(A3,1)+MIN(TIME(8,0,0)+E3,TIME(20,0,0)),"")
Note that for durations of more than 36 hours more days will need to be added to the three modeled in the attached file.
Let us know if you have any questions.