DST pair drift

When does the gap shift?

The hour-difference between two cities is rarely constant year-round. Pick a pair below to see exactly which weeks the gap changes — and why.

How DST-pair drift works

What a DST-pair comparison shows

A DST-pair page exists for one reason: the hour-difference between two cities is rarely constant. Each part below explains the generic mechanism; pick a pair above for that pair's exact week-by-week drift calendar.

Two zones with different DST rules

A DST pair is any two cities whose daylight-saving rules do not match — because one observes DST and the other does not, or because both observe it but switch on different dates. The hour-difference between such a pair is not a single number you can memorize; it depends on the time of year.

When the offset between them changes

The gap shifts on each transition date on either side. When one city springs forward or falls back, the hour-difference moves by one hour until the other city catches up. A pair where both observe DST can have three or four distinct gap values across a single year.

The desync windows

Between one city's transition and the other's, the pair sits in a temporary desync window — usually a few weeks in March and again in October or November — where the gap differs from its usual value. This is when recurring meetings silently move for one side.

Why a fixed offset fails

Scheduling a cross-zone call from a remembered hour gap breaks during every desync window. A DST-pair page shows the full year of gap values and the exact weeks they change, so a recurring slot can be set against an instant and a zone rather than a stale offset.

The fix for drift is the same everywhere: anchor a recurring slot to a universal-time instant plus an IANA zone, never to a remembered hour gap. The per-pair pages list every gap value and the weeks it applies so you can confirm both sides.

Reference data for DST-pair drift

Reference fields include the DST-pair comparison model, the reason the hour-difference drifts, daylight-saving transition dates, the desync windows between transitions, and the recurring-meeting drift problem.

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Questions

Frequently asked

What is a DST pair?: A DST pair is two cities whose daylight-saving rules differ — one observes DST and the other does not, or both observe it but switch on different dates. As a result, the hour-difference between them is not constant year-round.
Why does the hour-difference between two cities change?: It changes on each daylight-saving transition. When one city moves its clocks but the other has not yet, the gap shifts by one hour until the second city follows. A pair where both observe DST can show three or four different gap values across the year.
What is a desync window?: A desync window is the stretch of weeks — typically in March and again in October or November — between one city's DST transition and the other's, when the hour-difference temporarily differs from its usual value. Recurring meetings fixed in one city's clock appear to move during these windows.

What a DST-pair comparison shows

Two zones with different DST rules

When the offset between them changes

The desync windows

Why a fixed offset fails

Frequently asked

What is a DST pair?

A DST pair is two cities whose daylight-saving rules differ — one observes DST and the other does not, or both observe it but switch on different dates. As a result, the hour-difference between them is not constant year-round.

Why does the hour-difference between two cities change?

It changes on each daylight-saving transition. When one city moves its clocks but the other has not yet, the gap shifts by one hour until the second city follows. A pair where both observe DST can show three or four different gap values across the year.

What is a desync window?

A desync window is the stretch of weeks — typically in March and again in October or November — between one city's DST transition and the other's, when the hour-difference temporarily differs from its usual value. Recurring meetings fixed in one city's clock appear to move during these windows.

When does the gap shift?

What a DST-pair comparison shows

Two zones with different DST rules

When the offset between them changes

The desync windows

Why a fixed offset fails

Frequently asked

Never miss a launch, finale, or eclipse.

When does the gap shift?

What a DST-pair comparison shows

Two zones with different DST rules

When the offset between them changes

The desync windows

Why a fixed offset fails

Frequently asked