Assuming you are asking about entering information in the presence calculator:
A trip abroad begins with the date of exit (departure) from Canada, and ends on the date of return, the date the applicant actually arrives back in Canada. Those are the only dates that need to be entered.
Whatever the applicant did between those dates, that is all part of that one trip.
Beyond that, how a particular applicant populates the other boxes depends on the particular details and the applicant's best judgment about what needs to be disclosed and how to disclose it. Be responsive and truthful is the main guideline.
In the "destination" box, for example, if the trip was directly to one country and then directly back to Canada, there is a specific right answer, that country.
But otherwise, if more than one country was visited between the reported date of exit and the date of return, the applicant uses his or her own judgment about which of those countries should be listed in the "destination" field, leaving the other countries to be referenced in the "reason" box. My sense is that it is best to enter the country where the individual spent the most time, the primary destination. But other approaches may be just as OK. Again, this is not governed by technicalities, the "right answer" is a responsive and truthful answer.
Then, as noted, in the "reasons" box enter the purpose of the trip of course ("holidays" suffices, if indeed the trip was for holidays; "attend to unfinished business" works IF that was the reason; "visit ill parent" works if that was in fact the reason; and so on, and "to temporarily live in home country" works if that is what it was about), and then list all other countries visited during that trip. For a layover in London, just listing England or the UK "as additional countries visited" should do it. No need to elaborate; no need to distinguish a layover from longer stays. One could, of course, but better to keep it focused, precise.
