If you are trying to record the date/time any time cell C13 changes to "Y",
then paste this code into that worksheet's code module (right-click on the
worksheet's name tab, select "View Code"):

Private Sub Worksheet_Change(ByVal Target As Range)
'
' This prevents recursion into this subroutine -
' set it back to True before you exit!
'
Application.EnableEvents = False
'
If (Target.Row = 13 And Target.Column = 3 And Target = "Y") Then
ActiveSheet.Cells(16, 3) = Format(Now(), "mmm dd, yyyy h:mm AMPM;@")
End If
'
Application.EnableEvents = True
End Sub

Note: set cell C16 format to "Text", note "Date" or the VBA Format
statement will not make any difference!

HTH,

Eric