if then statement code

I am trying to write a statemtn in access that will determine if a date is null and if it is respond with “0”, if it is true, I want to be able to count the days (datediff) between it and a second date. If the second date is null, I would like to count the days from the first date to “today”. I also have a re-start and re-end date that I would like to incorporate and count all days. I would love any feed back and direction possible.