US Customs in Canadian airports: rules about timing?

Post date: 2018-10-13 04:16:17

Is there a rule that one must clear U.S. Customs in Canadian airports some minimum time before one's scheduled departure time, even if one's flight has already been delayed?

Context: when flying from most Canadian airports to the U.S. direct, one clears U.S. customs and immigration in the Ottawa airport before getting on the plane. This is awesome. My question is about whether rules exist for the hour at which one must clear U.S. customs.

I got the following notifications from United:

United Alert 1 of 2
Your 700am United flight to (X) is delayed because of (blabla). UAxxxx now departs Ottawa at 1134am and arrives at (hour)

2 of 2 United Alert
Due to Customs hours you must arrive at YOW at least 1 hour prior to your 7:00am scheduled dprt time

And then a few hours later (I had apparently been rebooked automatically in the meantime):

United Alert 1 of 2
Your 600am United flight to (Y) is delayed because of (blabla). UAyyyy now departs Ottawa at 1100am and arrives at (hour)

2 of 2 United Alert
Due to Customs hours you must arrive at YOW at least 1 hour prior to your 6:00am scheduled dprt time

What gives? Is it really required to arrive at the airport at least one hour prior to one's scheduled departure time in order to pass through U.S. Customs, when one already knows that one's flight is delayed five hours? Is there some rule about the minimum time-before-flight that the customs people will process you that short-sightedly does not take into account flight delays? (I can imagine there being such a restriction if one is so late that one would miss one's flight.)

Possibly relevant: U.S. Customs in Ottawa opens at 5 a.m. and the lineups are not generally an issue.

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