Cronjob

I made one which runs a php scrip every 30 minutes and its working perfectly, iam just wondering when it is run, is at x:00 and x:30 or after 30minutes when i first added it. Cuz i want it to run at x:00 and x:30 so that users know when it will it happen.I also want to know how do i get a the time remaining till the cronjob is done, if this can't be done directly how do you get the time at asta host, i don't want the entire date i just want the minutes and if its >30 i subtract 30 from it, example if its 49, it is shown as 19.Finally after the number is gotten i want a countdown timer to begin and when it hits 0 the page refreshes, i have the java script for the countdown and refreshing but how do i get and send the number to it.

It's all logical, it's every half hour if you told it. So the time php tells you will be the same, hopefully to the second.

It does exactly what you tell it to do if you told it every 30 min it will do it every 30 min

No i know it does it every 30 minutes but when did it do it the first time? when i added the cronjob or at X:00 or X:30

On the half hour means at :30, every half hour means :00 and :30.