Police clearance certificates need to be issued EITHER:
-- After the last time you were in that country or territory; or
-- No more than 6 months before the date you sign your citizenship application
This is stated on the face of the adult citizenship application form, CIT 0002, below the box for entering name of country in response to Question 10.b)
It is also stated in the guide for completing the adult grant citizenship application, likewise in instructions for answering Question 10.b) . . . here:
https://www.canada.ca/en/immigration-refugees-citizenship/services/application/application-forms-guides/guide-0002-application-canadian-citizenship-under-subsection-5-1-adults-18-years-older.html#Step4
Reminder:
if in doubt, follow the instructions; otherwise, yep, follow the instructions.
Moreover, this is essentially the standard for PCCs submitted to IRCC in regards to applications generally.
Six month criteria is obviously as of the date of the application. There is no indication that IRCC rejects or returns citizenship applications if an attached PCC does not technically comply with this. Close likely gets a pass, at least in terms of passing the completeness screening so that the application will processed.
However, in the course of processing the citizenship application if IRCC is not satisfied with the PCC it may request the applicant to submit another, which would be non-routine and result in a longer processing time; also NOTE: IRCC can request applicants to submit PCCs or additional PCCs anytime during processing.
Date of mailing or sending the application is of little or no relevance. Dates in the application and presence calculation, including in particular the date the application is signed and dates establishing the eligibility period, must be consistent. As long as the application is sent in time to be received less than 90 days after the signature date that meets the criteria to pass the completeness screening (if not within the 90 period, the application is considered stale and is returned).