How to Convert Timestamps

Problem

You have a timestamp in one format (e.g., Unix epoch) and need it in another (e.g., ISO 8601 or local time).

Prerequisites

• Understanding of Unix timestamps
• Knowledge of ISO 8601 format

Steps

1. 1. Identify the input format

   Determine whether the timestamp is a Unix epoch (seconds or milliseconds since 1970-01-01), an ISO 8601 string, or a custom format. Unix timestamps in seconds are 10 digits; milliseconds are 13 digits. This distinction is critical.

   Input

   1697254800 (seconds)

   Output

   Unix epoch in seconds

2. 2. Convert to the target format

   Parse the timestamp into a date object, then format it as needed. For ISO 8601, use toISOString(). For human-readable, use locale-aware formatting. For Unix timestamp, use getTime()/1000 (seconds) or getTime() (milliseconds).

   Input

   1697254800 → ISO 8601

   Output

   2023-10-14T03:00:00.000Z

3. 3. Handle timezones correctly

   Unix timestamps are always UTC. When displaying in local time, convert explicitly. Beware of the system's local timezone affecting parsing of date strings. Always store and transmit timestamps in UTC to avoid ambiguity.

   Input

   UTC → local time

   Output

   timezone-aware display

Expected result

The timestamp converted to the desired format, with timezone handling correct.

Common errors

Confusing seconds and milliseconds

Unix timestamps can be in seconds (10 digits) or milliseconds (13 digits). Check the magnitude and convert accordingly. JavaScript uses milliseconds.

Timezone offset applied incorrectly

Always work in UTC internally and convert to local time only for display. Use explicit timezone libraries for reliable conversion.

Troubleshooting

• If the date is off by hours, check for a timezone mismatch between the source and your system.
• If the year is 1970, the timestamp was treated as 0 or parsed incorrectly.

Security limits

• Timestamp conversion is a pure calculation with no security implications.