I suggest that you make the tables use the correct data type. It is really bad that you are using varchar for all of them. DB's are designed to work well with dates but if you set the data type to varchar you are basically shooting yourself in the foot. Check the MySQL manual and alter your tables to take dates. Once you do that repost if you have any questions.
As long as you are writing your thread in the PHP forum, then you should learn about these functions in php: (date,time,mktime) then change the varchar to int, this is easier and faster for your operations