Google Cloud Scheduler trigger last day of month

Question

I want to create a trigger in Google Cloud Scheduler that runs at 9am on the 25th and the last day of each month (depending of the month that would be 28, 30, or 31th).

I assumed somethink like this might work, but GCP does not understand the L-syntax:
0 9 25,L * *

Any (elegant) ideas how to do it without having multiple triggers?

Answer 1

One trigger, with minimized overhead calls:

0 0 25,28-31 * *

Then, inside the function:

IF is25() OR islastDayOfMonthHelper()
  work
ELSE 
  return