Auto mail after 24 hrs and mail by category

Hi, I need a script that send mail every 24 hrs if there is data that is only 24 hrs old in DB. And thees emails should bee sent to comanys in a serten category. I'l try to explain my site functions.

In one page i have a form whare users can submit job ads, the info inserted in this form is stored in DB under the table "jobadd" the main fields in this table to bee used with this function i belive are: "Time_Of_Submission"(the time when the data whas stored), "Vad"(what category this ad is in, ex, Cleaning service),

And on one page i have Signup form whare Companys can Register, and hear they can pick what Category is most suiteble for them ex, Cleaning service.
So hear all the submitted data is also stored in DB and table USERS. Hear the main fields to use are: email(the email address to registrated Company), Kategori(what category this Company is in, ex, Cleaning service),

Now i need to send mail to the Companys that is in the table "USERS" and the Category ex, Cleaning service. And if the data in "jobadd" and Category ex, Cleaning service is no older than 24 hrs.

Of corse i have many Catygorys not only Cleaning service. This is way i need this script to check what Category this data is in (if you know what i meen?).

But this could also work if the script only picks one Category ex, Cleaning service. This could work, for i'm planing to run this script with a cron job that i run from my Webhost controll panel. I just make a copy of this script changing the Category in every file to what ever Categoys i have.

I'm just not shore if it works to run maybe 20 cron jobs at the same time.
In the cron setup page on Webhost i can not set a specific time only pick once a day. and then the script will bee runed at midnight.

If anyboudy could help whit this (the PHP/MYSQL Code that is, it would bee great.

Thank you.

/Patrik.