Sharing availability across time zones without mistakes

Almost every scheduling disaster is a timezone disaster. "3 PM" means nothing without a zone; daylight saving shifts the offset twice a year on different dates in different countries; and mental arithmetic under a full inbox is how a Kentucky 3 PM becomes a London 3 PM.

Rules that prevent the mess

State the timezone once, at the top, and never mix zones in one list. Convert to the recipient's zone — it's courteous and it removes their chance to miscalculate. And never do the conversion in your head across a daylight-saving boundary; the offset you memorized may be wrong for the specific week you're scheduling.

The already-converted list

OpenSlot does the conversion at generation time. Pick any IANA timezone — the recipient's, not yours — and every slot is computed in it, DST handled, with the zone declared on line one:

All times in US/Eastern
• Today 07/08
  03:30 PM - 04:00 PM
  05:00 PM - 05:30 PM
• Thursday 07/09
  09:30 AM - 10:30 AM
  12:00 PM - 01:00 PM
  02:30 PM - 03:00 PM

Because the source is live Outlook free/busy rather than your memory of the calendar, the list is correct for the actual dates involved, including the weeks where US and European clocks change on different Sundays. Scheduling across a 9-hour gap becomes: pick their zone, generate, paste.

Why the offset you remember is sometimes wrong

New York is five hours behind London — except when it is four. The two sides do not change clocks on the same day. The United States springs forward on the second Sunday in March and falls back on the first Sunday in November; the European Union and the UK change on the last Sunday in March and the last Sunday in October.

That leaves roughly three weeks every spring, and about one week every autumn, when the London–New York gap is four hours rather than the five everyone has memorised. If you are booking three weeks out in early March, the offset you are doing the arithmetic with is not the offset that will apply on the day. The southern hemisphere makes it worse in the other direction: Sydney is moving the opposite way to both of them, so a London–Sydney gap swings between nine, ten and eleven hours across the year.

This is the single strongest argument for never converting by hand. The offset is not a property of the two cities; it is a property of the two cities on a specific date, and the date you are scheduling is not today.

Name the zone, not the offset

"EST" is the other reliable way to get this wrong. Half the year the US east coast is not on EST at all — it is on EDT — so a list headed "3 PM EST" in July is internally contradictory, and recipients resolve that contradiction differently. Worse, several abbreviations are ambiguous worldwide: CST is used for both US Central and China Standard, which are thirteen or fourteen hours apart.

Zone names of the IANA form — America/New_York, Europe/London, Asia/Kolkata — do not have this problem. They name a place whose rules are known, including when those rules changed, so they resolve to exactly one offset for any given instant. That is what OpenSlot computes against, and what it prints on the first line.

What converting to their zone actually moves

It is not only the clock times that shift. The day shifts with them, and so does everything keyed to the day. A Friday 6 PM in New York is Saturday morning in Sydney, so with weekends excluded that slot correctly disappears from a list built in Sydney's zone. OpenSlot applies the weekend test and your working hours in both zones, yours and theirs, so the list never offers their Saturday or your 3 AM.

Midnight moves too. A run of free time that crosses local midnight is split at the date boundary rather than printed as one line, because "11:00 PM – 2:00 AM" under a single day heading is the kind of line that gets a meeting booked on the wrong date. Day headings and midnight boundaries are computed in the recipient's zone, which is the whole point of picking it. If your working days barely overlap, widen the day rather than giving up: the list shows exactly the stretch that fits both.

Common questions