Selected Answer
It's hard to tell what you need but I guess it is a nested IF statement in a column that defines the beginning of a project's time calculation. Something like
= IF([PredecessorEndDate], IF([Duration], [PredecessorEndDate] + 1, 0), "Can't start")
In plain words, If there is an end date for the predecessor, check the Duration and set the start date (where the formula is located) to the predecessor's end date plus 1 day. Else, if there is no end date, write "Can't start".
The peculiarities here twice used is that (a) Excel evaluates any number to True and zero to False, and that a date is a number (if properly entered). Therefore, IF([PredecessorEndDate] and IF([Duration] would both be False if the referenced cells are empty, meaning that the project could also not start if no duration has been set for it. This condition would result in a zero entered as [StartDate]. You could use "" instead, but I prefer 0 because Excel is about numbers. You can format the cell to hide the zero and show a blank or even to display "Enter duration" when the value is zero, while other formulas referring to the start date could be made to take their lead from the zero value with similar IF conditions.
I hope this helps a little.