ガイド
JavaScriptでタイムスタンプ表示がずれる理由
秒・ミリ秒の混同やタイムゾーン設定による日付ずれを解消します。
Dateオブジェクト自体より、入力単位や表示方法の違いが原因になるケースが多いです。
JavaScript Date expects milliseconds
new Date(value) expects milliseconds in most timestamp workflows.
Passing seconds directly creates incorrect output.
Local time vs UTC confusion
Some methods show local timezone by default.
Use UTC-aware formatting when you need environment-independent results.
Debug checklist
Follow this order to isolate the issue quickly.
- Confirm unit length.
- Convert to UTC and local for comparison.
- Check source API timezone assumptions.
Avoid hidden string parsing issues
Different date string formats can parse inconsistently.
Prefer numeric timestamps or ISO-8601 strings for predictable behavior.
Typical problems
- Date near 1970 appears unexpectedly.
- Correct day but wrong hour.
- Different time in frontend vs backend.
- Broken date formatting in UI.
Debug unit first, timezone second
Most JavaScript timestamp bugs are solved by checking units and then checking timezone conversion.