Yes, sure. For this purpose, you can use one of Adaxes virtual properties that can store date/time values, for example, CustomAttributeDate1. Adaxes virtual properties are not stored in Active Directory, but can be used as any other properties of Active Directory objects.
You can add the property to the pages used for creating, viewing and editing users. For instructions on how to do this, see the 6th step of the following tutorial: http://www.adaxes.com/tutorials_WebInte ... diting.htm. After you press the Add button, you will need to select the Show all properties option as virtual properties are not shown by default.
Also, probably a name like CustomAttributeDate1 will not tell much to your users about the function of the property. You can give the property your own name. For instructions, see the following help article: http://www.adaxes.com/help/?HowDoI.Mana ... Names.html.