RTC in Micrologix

Hi, I am trying to achieve temperature control with Micrologix 1400. 12 fans starters. the lead fan starters will change every week to achieve uniform wear and tear.

week 1 - fan 1 will start.. run till 12
week 2 - fan 2 will lead.. go on till fan 1
week 3 - fan 3... fan 2
.
.
week 12 fan 12 to lead.. till 11.. and so on

Also the fans must drop out when the temperature matches the set point.

I am stuck here because, there is no week in the RTC. there are year, month, day etc..

RTC:0.YR
RTC:0.MON
RTC:0.DAY
RTC:0.HR
RTC:0.MIN
RTC:0.SEC
RTC:0.DOW (Day of week)

could someone please advice? what logic can I apply? Thank you.

When RTC.0.DOW=0 increment a counter. The counter will increment every Sunday.

Paul