When I worked at Network Appliance (Q1 earnings call is August 27th), they used a strict 4-4-5 calendar. https://en.wikipedia.org/wiki/4–4–5_calendar
This meant that it wasn't "about" 13 weeks for each quarter, the quarter was defined as 13 weeks (91 days).
Please add an offset functionality to your free solution immediately, as it has now become a core component of our operation, or we will be forced to take legal action.
Also, we appreciate if you could sign a retroactive NDA with our legal team ASAP.
This wasn't for any corporatey reason though; it just happened that "every 3 months" is a natural cadence for doing releases. (In a sense it's a 6-month cadence, like e.g. Ubuntu, but we support two major versions at any given time, and their schedules interleave.)
Sometimes a calendar quarter is just a calendar quarter.