Bob,
     There are several ways you can capture the date & time into a field. But first I want to clarify  that you do not have to have a real time clock to do so. I mainly use a real time clock to perform calculations on dates and times I have stored. IE: to determine the expiration date of an item. So enough of me talking, here is the example.

Im assuming,

1. Signed_out is a text box on a form bound to a table
2. Date&Time is your real time clock

In the "On click" event of your button put this.

If IsNull([Signed_out]) Then
[Signed_out].Value = [Date&Time]
DoCmd.Save

Else
DoCmd.Save

End If
End Sub

Basically what is happening, when you click the save button it checks the Signed_out text box for a value, if it contains a value it does nothing and saves the record. However if it does not contain a value, the date and time are entered into the field and the form is saved.

I did it like this because if I go back to edit my form and I save my record, I do not want to overwrite my data with a new date & time.

I Hope this helps.

Gary