When you log in to your account there are two date columns for your activity - the processed date and the activity date. Whilst I have had flights take a few weeks to credit, the activity date has always been "backdated" to the correct date. Only when one missing flight was credited manually has the activity date not backdated. That was put on as a goodwill credit though, so not even as a proper flight activity. I'd suggest that flights that don't backdate are definitely a small exception and not the norm. My experience is with flights on DJ, DL, and VX.
It is the activity date column that determines which frequent flyer year it credits to, not the processed date.
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