I would advise using a database (take a look at MySQL it's very easy and well documented). Store the date in timestamp mode (you can get a timestamp from strtotime()--just make sure to put it at the same time every time, so you have something to compare it with) and then select from a database using this time (of the current day).
For example, I enter: "5 May 2003", you want to strtotime() this at a certain time, let's say midnight (0:00), so you can use "0:00 5 may 2003" (and it should recognize this). Then, when checking the database, you want to select all rows with a date matching the current day's date, so look at mktime().
Note that using a timestamp should rule out all error in how they enter the date.