Once you create the check box, remember to assign the link to a
cell in the sheet. I chose E1 for the example
(I normally hide this value behind the CheckBox, out of site.)
Right click/format/control tab/link cell

Here is the simple code to do what I think you want:
Sub CheckBox1_Click()
If Sheets("sheet2").Range("E1").Value = True Then
Sheets("sheet1").Range("A1").Value = Now()
Else
Sheets("sheet1").Range("A1").Value = Empty
End If
End Sub