Be explicit

Be explicit: Don't use ambiguous date formats, "October 4th" is perfectly fine, "10/4" is stupid.

Be explicit: Not AM and PM, but 24-hour clock ("military time")

Be explicit: Don't use "GMT" for "UTC" ("London") time, we are at BST right now

Be explicit: Use UTC offsets for timezones (UTC-0400), or just specify the location (like America/New_York)