Yes, a timestamp is by far the best method. You can even set up mySQL to automatically update your timestamp for you, by using the TIMESTAMP column type: "The TIMESTAMP column type provides a type that you can use to automatically mark INSERT or UPDATE operations with the current date and time. If you have multiple TIMESTAMP columns, only the first one is updated automatically.

Automatic updating of the first TIMESTAMP column occurs under any of the following conditions:

* The column is not specified explicitly in an INSERT or LOAD DATA INFILE statement.
* The column is not specified explicitly in an UPDATE statement and some other column changes value. (Note that an UPDATE that sets a column to the value it already has will not cause the TIMESTAMP column to be updated, because if you set a column to its current value, MySQL ignores the update for efficiency.)
* You explicitly set the TIMESTAMP column to NULL.

TIMESTAMP columns other than the first may also be set to the current date and time. Just set the column to NULL, or to NOW(). "

All you have to do is store the time you last checked the database in a text file, and update the text file with the new timestamp whenever you make a check.