2 more, the simplest one first as you already have the times at the top of the sheet.
Excel Formula:
=IF(AND(C4>=$D$2, C4<=$E$2),"Day","Night")
FYI, times are decimal values, 1 hour is 1/24 days so you were actually looking for 7 and 19 days with your formula instead of hours. Dividing by 24 will covert any integer to hours
Excel Formula:
=IF(AND(C4>=(7/24), C4<=(19/24)),"Day","Night")
or the reverse can be done by multiplying (as in
@Fluff's suggestion).
As you can see from the answers provided, there are many ways to convert to time when needed.