Custom Auto Number Updates all previous numbers

I have a form Named "Shop_Tickets", with a sub form "Work_Orders".

The WO# field is the output of the following script on the sub form "On Add Row" work flow:

counter = "WO" + "-" + zoho.currentdate;

counter_num = 0;

counter_job = Jobs[ID == input.Job].Job_Number;

for each line in Work_Orders

{

counter_num = (counter_num + 1);

line.WO = ((((counter) + "-") + (counter_job)) + "-") + (counter_num);

}

which is producing exactly the results I want in the format I want, However the date is not staying persistent.

Example:

Monday 1/1/2015 - I add three WO's to the "Shop_Tickets" form of Job x

the wo#'s would be:

WO-1.1.15-51523-1

WO-1.1.15-51523-2

WO-1.1.15-51523-3

Tuesday 2.1.15 - I add one WO to the same " Shop_Tickets" form

WO-2.1.15-51523-4

which is the correct number and format, but it changes the numbers on all the previous tickets to:

WO-2.1.15-51523-1

WO-2.1.15-51523-2

WO-2.1.15-51523-3

so every time i add a row it is updating the current date, of that field in every row.

Am I just missing the way to prevent this?