The APUS Student Handbook is an online source, so you should cite it according to your program's style guide instructions for a web document.
Because the Student Handbook is updated frequently (sometimes several times a week), it is, in essence, a living document. So there is no single date for the entire Handbook. (If an actual policy changes, an effective date is placed next to this information. Most other changes are clarifications and process changes).
Different style guides handle the issue of “no date” differently. Some recommend using “n.d.” where the date would be in a citation. Some state that missing information should just be left out. Consult your program’s style guide for the correct usage.