Simple Staff cost budget - 12 months forecast

Hi All,

I need some help. Which formula would i use to:

If start date cell is blank = calculate a monthly salary
If end date cell is blank = calculate a monthly salary
If start date has a date = calculate only from the start date and every month onwards
IF end date has a date = calculate only up to end date.

I used to have this formula but i have lost the spreadsheet moving jobs.

Can someone please help me.