While there is only one logical way to represent an absolute moment in the time (and hence only one wxDateTime class), there are at least two methods to describe a time interval.
First, there is the direct and self-explaining way implemented by wxTimeSpan: it is just a difference in milliseconds between two moments in time. Adding or subtracting such an interval to wxDateTime is always well-defined and is a fast operation.
But in the daily life other, calendar-dependent time interval specifications are used. For example, `one month later' is commonly used. However, it is clear that this is not the same as wxTimeSpan of seconds because `one month later' Feb 15 is Mar 15 and not Mar 17 or Mar 16 (depending on whether the year is leap or not).
This is why there is another class for representing such intervals called wxDateSpan. It handles these sort of operations in the most natural way possible, but note that manipulating with intervals of this kind is not always well-defined. Consider, for example, Jan 31 + `1 month': this will give Feb 28 (or 29), i.e. the last day of February and not the non-existent Feb 31. Of course, this is what is usually wanted, but you still might be surprised to notice that now subtracting back the same interval from Feb 28 will result in Jan 28 and not Jan 31 we started with!
So, unless you plan to implement some kind of natural language parsing in the program, you should probably use wxTimeSpan instead of wxDateSpan (which is also more efficient). However, wxDateSpan may be very useful in situations when you do need to understand what `in a month' means (of course, it is just wxDateTime::Now() + wxDateSpan::Month()).