Help with a Formula 'Isblank refer to another cell"

I need a formula that will give me number of days between two columns Which contain dates, if one or the other has no date populated it would then refer to another cell which also should contains dates. If the difference is a negative number, should have text saying "dates out of sequence". I hope I am making sense.
There are actually 4 columns:
lets say A,B,C,D; the column titles are 095 Actual Date (A), 095 Forecast Date(B), 126 Actual Date(C) and 126 Forecast Date(D).
So its the difference between 095 & 126 columns.
The actual date is the 1st query and if there are no dates populated then should got to the Forecast date column.
Not sure if this makes sense to anyone! Please help..