Hi, and welcome to the forum.
No 1.
A2: =IF(DAY(A1)<=15,A1+32,A1+64)
formatted as "mmmm"
No 2.
Assuming you've got Start & Finish times in A1:B4, with C1:C4 being the difference between the two times (formatted as a number). Then use the formula
=SUMIF(C1:C4,">1",C1:C4)
cells in C1:C4 with error values will not affect the calculation.
HTH
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