Date
By default: July 16, 2021 (write dates as "month day, year")
Friday, July 16, 2021 (if you need to include the day of the week)
Fri, Jul 16, 2021 (when lacking space)
Time
By default, use the 24-hour clock: 12:55 or 9:24 or 10:00 (no zero at the beginning for the morning times)
If you really need to use 12-hour clock format, use AM and PM: 9:42 AM or 12:43 PM or 10 PM
When describing a length of time, use abbreviations: 12h 23min.
Date & time together
When describing the date and time together, start with the date and separate it from the time with a comma: July 16, 2021, 10:35 AM
Specific time versus ago
# <1 hour: X minutes ago
# e.g. 59 minutes ago
#
# today: HH:MM
# e.g. 12:10
#
# past, future: Month, day, time
# e.g. Jul 26 12:10
Abbreviations
Month/months: mo/mos
Minute/minutes: min/mins