Mdl script to automate changing date range in Dimension

I have the following script that updates the earliestdate in my Model file (pyj or mdl). The problem is I have >50 model files I need to update these dates on and I have to update them quarterly changing the earliest and latestdate by 3 months each quarter.
So currently it is set to 20150401 thru 20170331

Next month I will need this to shift forward to 20150701 thru 201700630

Here is my current code to automatically change the dates:
I am currently using Cognos 10.2.1.
I am looking for a way to pass a value based off of today’s date through to my code below:

 OpenPy "\\path_to_my_file\filename.pyj"
 DimUpdate "DimensionName"
 DimType Date EarliestDate 20150701
 DimType Date LatestDate 20170630
 SavePy "\\path_to_my_file\filename.pyj"

I cannot figure out IBM’s language to do this. I have searched high/low on IBM’s site but cannot seem to figure out an automated way to do this.

Ultimately something like this; but %let does not work:

 %let earlydate=today()-730;
 %let late date=today()
 OpenPy "\\path_to_my_file\filename.pyj"
 DimUpdate "DimensionName"
 DimType Date EarliestDate &earlyldate 
 DimType Date LatestDate &latedate
 SavePy "\\path_to_my_file\filename.pyj"

Any ideas?