Timestamp trouble.

[Beau Beveridge]

I'm using PHP 4.3.8 and MySQL 3.23.58. I'm collecting data from an HTML form which then uses PHP to insert the information into a table in MySQL. In the table I have a field that's type is TIMESTAMP. My problem is that it seems no matter what I try that field is always entered as 00000000000000. The field is set as NULL; I thought it was supposed to be NOT NULL but it won't let me change it. Using PHP I've tried sending nothing (''), a now() function and entering the info manually but I can't figure out what I'm supposed to be doing.