I need a formula which details if we are overdue on completing a task.

I'm trying to create something which will provide me with a +/- number of days we are overdue (or left remaining)

S7 = Due Date
T7 = Date Sent
U7 - Days Remaining

I need todays date, minus S7(Due Date) but only if T7 is blank. If T7(Date Sent) has a date entered, then I need U7(Days Remaining) to be S7-T7

*I think*

Is this even possible?