Several ways to do this.

Event code can be tailored to suit your needs.

Where would you like to see the timestamp?

On one sheet or on all sheets?

This sheet event code would give you a timestamp in B1 whenever A1 was changed.

I'm sure you have something more complex in mind, but example only..

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$A$1" Then
Target.Offset(0, 1).Value = Now()
End If
End Sub

Gord Dibben  MS Excel MVP