Depends on how you've got it set up, and there are numerous ways to do it. Presuming it's a form or subform based on a query that uses that textbox in its criteria, one way would be to add this to the code behind the OK button on his calendar:
Code:
Private Sub cmdOk_Click()
On Error Resume Next
'Purpose: Transfer the result back to the calling text box (if there is one), and close.
If Me.cmdOk.Enabled Then
If gtxtCalTarget = Me.txtDate Then
'do nothing
Else
gtxtCalTarget = Me.txtDate
Forms!YourFormName.Requery
End If
End If
gtxtCalTarget.SetFocus
DoCmd.Close acForm, Me.Name, acSaveNo
End Sub