Sometime around last spring we needed to implement time based
scheduling for tasks at $JOB. As we were all seasoned cron
veterans, we went with adding cron-like expression support to our
orchestration/background processing engine. I implemented it, added
some tests on top, my colleague tested it and everything was fine.
Fast forward to November, one of the tests I added started failing
with "ArgumentError (mon out of range)". The test was about testing
monthly scheduling. What I did in that test was:
1) Store current time
2) Disassemble the time to years, months, days and so on
3) Make month = (month + 1) % 12
4) Increase year if months "overflowed" to another year
5) Assemble new datetime
6) Compare the time when next execution should occur with time
constructed in 4)
At that time, it seemed like a perfectly sensible thing to do. Now
it shouldn't be hard to tell what went wrong. In November the "new
month" was 0 and the Date class expected a number between 1 and 12.
Sadly because the test started failing after almost half a year of
reliable service, we first started to look for changes in the
libraries we depended on. It took some time to realize, the only
thing changed was time.
