if the previous cell is blank then use the previous formula

[limbelah]

oh my god, my head is spinning like crazy now and I need help from you all. How do I key in the formula for the following cells :

1. FOR CELL D: I need to count the duration let's say from 5:30pm, Friday to 4:00pm, Monday in Cell D but I have Sunday not filled in...
2. FOR CELL E: I need to count the usage but again, my Sunday is not filled in..
3. FOR CELL G: I need to count the Usage Amount but again, my Sunday is not filled in...

I want to keep the Sunday there, in case, I have the date to be filled in once in a while... and also, I want to leave the cell blank if there's no data on Sunday...

Please help, I can't thank you guys enough...