dateutil is an extension package to the python standard datetime module. As you say, it provides extra functionality, such as timedeltas that are expressed in units larger than a day.
This is useful if you have to ask questions such as how many months can I save before my girlfriend’s birthday comes up, or what’s the last Friday in the month? This hides complex calculations that are caused by the different lengths of the months, or extra days in leap years.
In your case, you are only interested in the number of days. So you’d best use timedelta as this avoids an extra dependency on the dateutil package.