How to count the time?

[Eric]

Does anyone have any suggestions on how to count the time?
Lunch break is between 12:30 pm and 2:30 pm

Example 1
I would like to manage and know when I should start working on the task, if
I need 1 hour to finish A project, and the deadline is 3pm, therefore I
should start work on it at 12pm, 12:00 - 12:30 and 2:30 - 3:00 = 1 hour

Finish time is given at cell C3, which is 3 pm
and the required time is given in cell C6, which is 60 minutes
The starting time sould be returned in cell C2, which should be 12 pm

Example 2
I would like to manage and know when I should start working on the task, if
I need 30 minutes to finish A project, and the deadline is 2:45pm, therefore
I should start work on it at 12:15pm, 12:15 - 12:30 and 2:30 - 2:45 = 30 mins

Finish time is given at cell C3, which is 2:45 pm
and the required time is given in cell C6, which is 30 minutes
The starting time sould be returned in cell C2, which should be 12:15 pm

Does anyone have any suggestions on how to do it in Excel?
Thanks in advance for any suggestions
Eric

 

[Luke M]

Assume that you have beginning of lunch (12:30 PM) in cell A2, and end of
lunch (2:30 PM) in cell A3. Also, time duration in cell C6 is written as time
(60 minutes is inputted as 1:00 or 0:60).

Formula is:
=IF(C3-C6<A3,A2-(C3-A3),C3-C6)