The ISO 8601 standardizes the representation of dates and times, and these standard representations are often used to construct timestamp values. With respect to the time, the ISO 8601 uses the 24-hour notation with
: (colon) separators as
Therefore, the adhesion to
hh:mm could be considered a "best practice".
In LaTeX, package
datetime use this format as the default in
\currenttime. But with
\settimeformat you can change the style xxivtime to ampmtime to display the current time in the 12-hour format followed by ante meridiam/post meridiam abbreviation (6:00pm).
This is because on the contrary to the common format on digital clocks and computers, humans often like the 12-hour format with am/pm abbreviations because is simpler and unmistakable, so this, with or without ISO, is also a good practice. As pointed in the comments, this depends also of the country uses. This is explained with more detail this page of Wikipedia (but actually the list of countries where the 12-hour format is common seems incomplete).
Another options, for example in a invitation card, is to display the time as text (for current time, set format style to oclok)
For a 12-hour format, instead of ante meridiem abbreviation in lowercase (am) you may prefer "A.M." in small caps. In this case you can use hh:mm
\AM or hh:mm
\PM with the package
abbrevs (of course, you can make also a little macro, but these correctly handles following periods).
I do not recommend the period as separator (18.00 format) even if it usual in your country, as the time could be confused with a number, even if decimals are marked with a semicolon.
(for change it in
Finally, at least for me, 18ºº look unusual and I think that it could be confusing, mainly if there are near symbols of Celsius degree, superscripts with numbers (reference to footnotes, chemical compounds, equations, etc.) or when is not clarified by the context that 18ºº is the time. If the point is to make something original/elegant with the time, please consider use a font with old style numbers.