MySQL timestamp field update problem

Hi all, I have a table in MySQL with the following field: 'time_date' timestamp(14) NOT NULL When a record is inserted into the table, NULL is inserted into the time_date field giving me a normal timestamp with the time of the INSERT. That much is fine. However, it is necessary for me to update some of the records in my table. But whenever I send an UPDATE, the timestamp changes. I would like for the timestamp value to remain unchanged. Is there a way to do this without reading the old value of the timestamp and reinserting with my UPDATE query? Thanks. -Rob