What characters might be used to substitute for colon in folder names using a time-of-day?
ISO 8601 “Basic” variation
You can avoid using the colon by employing ISO 8601 “basic” format strings (YYYYMMDDTHHMMSSZ) for naming files and folders. The standard allows for the separators to be omitted from the YYYY-MM-DDTHH:MM:SSZ format.
So both of these examples are valid under the standard:

2016-03-27T07:01:02Z
20160327T070102Z

If you need to process those strings with certain programming or scripting languages then maybe you have to convert them to the extended format that incorporates separators (hyphens, colons) and time zone info.
International standard date and time notation - &quot;If a date and a time value are stored together in a single data field, then ISO 8601 suggests that they should be separated by a latin capital letter T, as in 19951231T235959&quot;. So the T in the middle is optional (19951231235959) but suggested.