タイムゾーン重複計算ツール（リモートチーム向け）

計算の仕組み

Each line in the box is an IANA zone name. For every zone the tool asks the browser's Intl.DateTimeFormat what the local time is right now, derives that zone's offset in minutes, and projects each local working hour to UTC as ((h − offset ÷ 60) mod 24). The overlap is the set of UTC hours every listed zone marks, and it is those UTC hours — not local ones — that the headline count and the coloured band report.

Work start hour and Work end hour are whole numbers in 24-hour form, defaulting to 9 and 17, and the window is half-open: 9 to 17 covers 09:00 through 16:59, eight hours. Widening by one hour at each end often rescues a dead result. Only the first eight lines are used, and the shipped list — Los Angeles, New York, London, Berlin, Singapore, Sydney — leaves no shared hour at all, which is a fair demonstration of the problem.

Two things to know. Offsets are read for today only, so a meeting scheduled across a daylight-saving changeover will drift by an hour and needs rechecking nearer the date. And zones are bucketed into whole UTC hours by rounding down, so half-hour and forty-five-minute offsets such as Asia/Kolkata, Australia/Adelaide and Asia/Kathmandu land up to fifty-nine minutes early. A misspelled zone name is skipped silently rather than flagged.

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