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The DATEDIFF() function was added in MySQL version 4.1.1. For earlier versions that do not support this function you can make a work-around for it using the TO_DAYS function:

SELECT TO_DAYS(date1) - TO_DAYS(date2);

-- example
SELECT TO_DAYS('2010-01-03') - TO_DAYS('2010-01-01'); -- returns '2'

This will give the same effect as DATEDIFF(date1, date2) by converting each date to the number of days since year 0 and subtracting them.

Note: According the MySQL manual, TO_DAYS() is not intended for use with dates that precede the year 1582 because it does not take into account the days that were lost when the Gregorian calendar was introduced so using this method is not advisable for dates earlier than this.