I think the issue is the due date for C corp tax returns changed back in 2016 or so. It used to be they were due 3/15 so 6 months afterwards was 9/15.
See this link
https://www.cpa-wfy.com/get-ready-businesses-some-filing-due-dates-are-changing/
I quote (bold mine):
For many years, C corporation federal income tax returns on Form 1120 were due two and a half months after the end of the corporation’s taxable year (March 15, adjusted for weekends and holidays, for a calendar-year corporation). Form 1120 could be automatically extended for six months (through September 15, adjusted for weekends and holidays, for a calendar-year corporation).
However, a law passed last year established new due dates for Form 1120. For tax years beginning after December 31, 2015, the due date is generally moved back one month to three and a half months after the close of the corporation’s tax year (to April 15, adjusted for weekends and holidays, for a calendar-year corporation).
So someone taught you a short hand version of the rule and never taught you why is was 3/15 and 9/15. So when the due date of the 1120 changed you didn't pick up on it changing the deduction deadline.
Zeller's way of saying it is the better way to teach it and that hasn't ever changed.