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.