I think they record the timezone and correct time/date, but I can't say for sure.1106. I was going to do 5 days buffer, but I noticed in the ATIP response they had several days as the day after, since all times were in EST and I arrived in MST many times. It was the day before in Calgary, though it appeared as the day after in Toronto/Ottawa.
I don't know how the law is interpreted, since I don't think the law says everyone exists in Ontario time, but I didn't want to risk it. I reported my days as the day I actually arrived in MST, not the EST time from ATIP.
I assume that is very common.
I wrote up what was in our CBSA atip response, the sheets they sent:
https://www.canadavisa.com/canada-immigration-discussion-board/threads/urgent-help-request-pr-online-renewal-time-spent-outside-of-canada-cannot-enter-same-day-trip.865911/post-11001279
If it helps, but I can't say it will be same for all. In some respects our case perhaps easier (all entries/exits by air from same airport).