When other, non-native English speakers were invited to hangouts they were even more confused—asking me what he meant & I would have to look it up. In casual speech or storytelling these things don’t matter but when planning events & meetings they do. I have seen so many confusing scheduling issues in work & life that can be solved by just communicating clearly & precisely. I have seen meetings missed for time zones & ambiguous phrases like “biweekly”. You know what I do? I send an clear date & timestamp + *.ics iCalender file since I try to put events in my calendar since I can be forgetful, & it is almost no effort to forward it to the other interested parties. The other end then has a precise reminder that can be localized/translated however is clear to them in their calendar—& as a result no one has mistaken an event.

Yes, the “obvious poor social skills” of being clear with folks when their time is involved. As well, trying to get someone else to give up their speech oddity in planning for the sake of everyone else, myself included, by explaining that others are confused & it not being worth it. Do you have experience working in international groups?

When you do not include a preposition like til or past or before or after there is no way to underestand relative to which side of the hour. This is why it is interpreted differently in some cultures. This is also why no one I grew up with ever said anything other than 5:30, 6:30 PM, or 17:3:0 since—aside from the 12-hour Anglophone clock thing—you can remove both ambiguity & doing mental math (also typing less characters).

Funny when I first read about it: https://en.m.wikivoyage.org/wiki/English_language_varieties#Date_and_time

Which had explicit instructions

Some of these can be made less ambiguous (for example, Americans usually say “quarter past eight” or “quarter till eight”) but others will always have the potential for confusion. Be prepared to clarify, or simply use explicit dates and times.