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Every timer stores its target as an absolute UTC timestamp. The countdown you see is always the difference between that target and the current time, computed fresh on every tick - never a stored counter that gets decremented.

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How remaining time is computed

When you create a timer, the date, time, and time zone you pick are converted to a UTC ISO 8601 timestamp once, at save time. From then on the remaining time is pure arithmetic, implemented by getCountdownParts in lib/utils.ts: 
const targetMs = new Date(targetDateIsoUtc).getTime()
const deltaMs = targetMs - nowMs
const isCountUp = deltaMs < 0
const totalMs = Math.abs(deltaMs)

const totalSeconds = Math.floor(totalMs / 1000)
const days = Math.floor(totalSeconds / 86400)
const hours = Math.floor((totalSeconds % 86400) / 3600)
const minutes = Math.floor((totalSeconds % 3600) / 60)
const seconds = totalSeconds % 60
The delta is split into days, hours, minutes, and seconds with integer division. Days are total days, not calendar months or weeks. When the target has passed, the same numbers are shown as a count-up (“since”) instead.

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Where “now” comes from

The current time is sampled by the useNow hook in components/use-now.ts: it reads Date.now() and re-samples it once per second with setInterval. Two practical consequences:
  • No drift. Because each tick recomputes target - now from absolute timestamps, a delayed tick cannot accumulate error. If the browser throttles timers in a background tab or the laptop sleeps, the display may pause, but the very next tick is exactly right again.
  • It trusts the device clock. Date.now() is the device’s wall clock. If the device clock is two minutes off, the displayed remaining time is two minutes off as well. There is no server time synchronization.

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Time zones and recurrence

Converting your wall-clock input (“June 30 at 09:00 in Europe/Warsaw”) to UTC uses the date-fns-tz library, which delegates to the browser’s built-in Intl.DateTimeFormat and its IANA time zone database. Daylight saving rules come from the browser, so they stay current with browser updates. Recurring timers store a wall-clock pattern in the timer’s time zone and compute the next occurrence on demand. Occurrences are calculated, never mutated, so the countdown is correct even if the app was closed while several occurrences passed.

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Browser support

APIUsed forSupport
Date.now()sampling the current timeevery browser since ES5
setIntervalthe 1-second tickuniversal
Intl.DateTimeFormat with IANA timeZonetime zone conversionall evergreen browsers
All three APIs are available in every current evergreen browser; the only one with a meaningful history of partial support is Intl.DateTimeFormat time zone handling in long-obsolete browsers. To verify support yourself: