A bulletproof time reference is one that cannot be misinterpreted regardless of where the reader is located. Writing one requires including three pieces of information: the time, the UTC offset, and a geographic anchor. Each element serves a different purpose.
The three components
The time itself: use 24-hour format where possible to eliminate AM/PM ambiguity. "14:00" is unambiguous. "2pm" requires the reader to know whether you are using a 12-hour clock, which is not universal.
The UTC offset: write the offset explicitly as UTC+X or UTC−X. Do not use an abbreviation. "UTC+5:30" is unambiguous. "IST" is not. Include the sign explicitly — "UTC+0" rather than "UTC".
The geographic anchor: include a city name as a human-readable cross-reference. City names are more stable than timezone names (which change with DST) and more recognisable to non-technical readers than UTC offsets alone. "14:00 UTC+5:30 (Mumbai)" gives the reader three independent ways to verify the time.
The full format
The recommended format for a bulletproof time reference in written communication is:
[Day, Date] at [HH:MM] [UTC±offset] ([City])
Example: Tuesday 10 March at 14:00 UTC+5:30 (Mumbai)
For audiences in multiple timezones, include each relevant local equivalent in parentheses:
Tuesday 10 March at 14:00 UTC+5:30 (Mumbai) — 09:30 UTC+1 (London) — 04:30 UTC−4 (New York)
What to avoid
Abbreviations alone: "3pm CST" is ambiguous. "3pm CST (Chicago)" is better. "15:00 UTC−6 (Chicago)" is bulletproof.
"Local time" without a reference: "the call is at 10am local time" tells a distributed audience nothing.
Relative phrases without anchors: "Tuesday morning" means different things to people in different timezones and becomes incorrect once the message crosses a date boundary.
Relying on calendar software to handle the conversion: calendar invites store times correctly when created carefully, but forwarded invites, copy-pasted times, and times mentioned in message bodies are not processed by calendar software and carry no timezone metadata.
The shareable link as an alternative
When writing a bulletproof time is not practical — in a casual Slack message, for example — a shareable link from a time interpretation tool is an equivalent alternative. Paste the time into TimeMeaning, resolve it, and share the canonical link. The recipient sees the full interpretation with all assumptions stated, regardless of their location.