Complex date equation - help needed.

Hi Guys, I need help with a complex date equation.
I'm trying to add "P6/W1" to a watchface, where P6 is the Period (There are 13 in a year) & W1 is the week in the period (there are four). The first day of the week is Sunday BTW
Period 1, week 1 started Sun 13th March 2016 and today (2nd Aug 16) is Period 6, week 1. There is a list of all the period start dates below.
If anyone can tell me what to type into the text field to end up with "P6/W1" I'd be really grateful.
Thanks.


Period Date Days
P1 13/03/2016 0
P2 10/04/2016 28
P3 08/05/2016 56
P4 05/06/2016 84
P5 03/07/2016 112
P6 31/07/2016 140
P7 28/08/2016 168
P8 25/09/2016 196
P9 23/10/2016 224
P10 20/11/2016 252
P11 18/12/2016 280
P12 15/01/2017 308
P13 12/02/2017 336
P1 12/03/2017 364

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