My curiosity got the better of me, so I looked through ip08 and found this:
5.3. Application made
Reference to an "application made" in the Regulations means the date that the application is date
stamped as received by the CPC-V.The CPC-V date stamps an application as received once they
have determined that the application is complete (see Section 5.4, below).
Under the spousal policy, many clients can benefit from an administrative deferral of removal if
there is evidence that they have a pending spousal application by the time they are deemed
removal-ready by the CBSA. In general, the date that the CPC-V has locked in the application is
the proof that an application has been made. For cases where a client attests that they have
made an application that has not been locked in, clients may present a copy of their application as
well as a copy of their fees receipt to show that an application has been made. Such proof may
also assist the CPC-V in locating the file for prompt action.
5.4. When does an application exist?
An application in the spouse or common-law partner in Canada class requires receipt by the
CPC-V of a properly completed and signed Application to Sponsor and Undertaking
[IMM 1344AE], a properly completed and signed In-Canada application for permanent resident
status [IMM 5002E] including the Background Declaration [IMM 5002E-Schedule 1] and proof of
payment of the correct processing fees. Under the spousal policy, H&C applications with a
spousal connection will be considered applications in the spouse or common-law partner in
Canada class after the receipt of a sponsorship, if not already submitted.
5.13. Lock-in age for dependent children
The age of any dependent children is locked in on the date the sponsorship and permanent
residence applications are jointly received, completed and signed, with the minimum requirements
met as specified in the Regulations and with proof of payment of the correct processing fees.
Maybe this means that CIC does in fact use the date received as their lock-in date.