You may also want to examine the documentation for the WEEKU, WEEKV and WEEKW formats for some of the issues involved with weeks and treatments at beginning and ending of years.
And when I look at a calendar between 13 Jan 2019 and 31 Jan 2019 I would expect to get 2 weeks. Which day of the week to expect the week to start on? SAS treats Sunday, which 13 Jan 2019 was, as the start of week, the next week starts on 20 Jan (1 week interval) and the then 27 Jan starts a week (2 week interval).
Note that intck returns intervals, or 1 for adjacent days such as 13Jan2019 and 14Jan2019. So if you need to count days including the end dates you need to add 1. Perhaps that is what you need to do with your definition/need of week counts.
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