Hmmm, you may have a problem with this.
=today() will always show the current date. So if you insert a new line(s) wiht the same formula you'll end up with all of your cells saying the same date.
Insert the following code in to a new module in your VBA editor:
ActiveCell.Value = Date
Assign a shortcut key to run the macro when you need to. Alternatively use this slightly modified version:
If .Value = Date Then
.Offset(-1, 0).Value = Date
This will check the last entry in the date list and check to see if it todays date. If it is the procedure ends. If not it will insert a line and enter todays date.
If you want the macro to tun when the workbook opens, insert the folling code into the 'This Workbook' page:
Private Sub Workbook_Open()
This basically means that when the workbook opens it will run the code above.