Restructuring a dataset

[SAS_user122]

Hi! I have a dataset that looks like this:

patient_id start_date     end_date

123           01Jan2016   15Apr2020

321           09Sep2018   17Nov2019

567            07Jul2017    09Jul2020

567            09Sep2020   10Dec2020

My study period is 01Jan2016-31Dec2020. I want to create a new variable called time that creates a flag if someone's study period included that particular month (ex. 1=Jan2016, 2=Feb2016, etc). So my data set would end up looking like this:

patient_id start_date     end_date      time

123           01Jan2016   15Apr2020    1

123           01Jan2016   15Apr2020    2

123           01Jan2016   15Apr2020    3

123           01Jan2016   15Apr2020    4

etc

Any thoughts on how to do this?

[PaigeMiller]

So what is the expected output for patient_id 321 and patient_id 567?

Is the first month always January 2016, or does it vary by patient_id?

What is the last month to use?