TODAY Function review/alert system

Hi

Im looking to create a tool that allows me on a particular day say (T) to type into a cell a command such as "T1", this would then return in another cell todays date plus 1 day. Likewise if I entered "T3" it would return todays date plus 3 days. Its basically a reviewing system.

So if today (T) is 20/10/11, and I enter "T3" in a cell, another cell would return "23/10/11"

..now I've actually managed to do this (which surprised me as I'm rubbish with excel), by using this:

=IF(E8="T1",TODAY()+1,IF(E8="T3",TODAY()+3,IF(E8="T7",TODAY()+7,"")))

You'll see I'm only bothered about T1, T3 and T7.

However the problem comes where I need an alert to come up when these "T" dates fall 'Today,'

Ie using the example above, if I enter a T3 command on a row today, I need to know about that entry through an alert on the 23/10/11.

I know that the reason the above wont allow me to do it is because the TODAYs will continually change to todays date, so the alerts would just roll on. So is there a way of fixing the 'T' dates to allow me to refer to them in the future?

Thanks

Joe