53 Week year effect on PRSI 352 threshold.

I have an employee who earns a total of 352 per week and as such is exempt from employees PRSI. However 2015 has 53 thursdays in it which means her annual salary is now
18656.00 as opposed to 18304.00
Welfare.ie clearly states PRSI is calculated on a weekly basis
However all the tax calculators suggest her salary has gone over the prsi ceiling.
I understand how to deal with PAYE and USC for the 53rd week but the PRSI isnt explained anywhere.

If you search for "week 1" basis I think that's the way to do the calculation for PRSI. Am on mobile browser so can't do the search myself.

From the DSP website:

"The maximum number of weeks of insurable employment that any employee can have in any income tax year is 52. However, if two paydays occur from the beginning of week 52 until the end of the income tax year, PRSI contributions will be paid on each payment as normal"