A Deliberate Browser Behavior, Not a Bug
Browsers intentionally slow down JavaScript timers in tabs you're not currently looking at — after roughly five minutes of inactivity, updates can be throttled to as infrequently as once per minute. This isn't an oversight; it's a deliberate tradeoff to save battery and CPU on tabs you're not actively watching, and every major browser does some version of it.
Why This Can Break a Naively Built Timer
A countdown timer that simply decrements a counter every time its internal tick fires — expecting exactly one tick per second, ticking down from 60 to 0 — will visibly fall behind real time once the tab is throttled in the background. Switch away for several minutes, switch back, and a naive implementation shows more time remaining than has actually passed.
The Correct Fix: Always Recalculate From the Real Target Time
A properly built countdown timer doesn't count ticks — it stores the actual target end time (a specific point in time), and on every tick, however delayed that tick might be, recalculates the true remaining time by comparing the current real time against that fixed target. Even if a tick is late or throttled, the moment it does fire, it corrects itself immediately to the accurate remaining duration, rather than compounding drift over time.
Why the Alarm Still Fires at the Right Moment
Because the underlying calculation is always based on real elapsed time rather than a naive tick count, a correctly implemented timer's sound alert triggers at the genuinely correct moment once the tab regains focus and its next tick fires — even if the display was briefly stale while the tab was in the background.
What This Means for You Practically
If you're relying on a countdown timer for something where precision genuinely matters, keeping the tab active (rather than in the background) during the final stretch avoids any chance of a delayed correction. For casual use, a well-built timer self-corrects the moment you switch back, so this rarely causes a real problem in practice.
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