How Computers Store and Manage Time

How Do Computers Store Time?

• Unix Time (Epoch Time)

• System Time

• File System Timestamps

• Network Time Protocol (NTP)

• Binary-Coded Decimal (BCD)

• System BIOS/UEFI Time

Unix Time (Epoch Time)

UNIX timestamp: the number of seconds that have elapsed since 1970-01-01 at 00:00:00 UTC

• Example:

  • Timestamp in seconds: 1695718262

  • Timestamp in milliseconds: 1695718262342

• Facts:

  • In older 32bit systems, the signed integer will overflow in 2038

  • Why 1970? The Unix OS was created in the late 1960s and early 1970s. Unix engineers arbitrarily selected 1970

Datetime Format

• ISO 8601: 2023‐09‐25T16:20:52+07:00, 2023‐09‐25T09:20:52Z

  • Z = +00:00

• Vietnam: 26/09/2023

• RFC 2822: Mon, 25 Sep 2023 09:16:58 +0000

• Javascript: YYYY-MM-DDTHH:mm:ss.sssZ

• Java: Tue Sep 26 16:05:37 ICT 2023 (+0700)

• RFC 3339

When to use DateTime, Timestamp?

• Timestamp: to record a (more or less) fixed point in time.

  • Example: created_at

• Datetime: time can be set and changed arbitrarily.

  • Example: schedule time for appointments

Best Practices

Backend uses UTC time zone + ISO 8601/timestamp

Frontend uses local datetime (show timezone)

DB stores as timestamp if possible

Store datetime with time zone

MySQL: use fractional seconds for accuracy

JVM timezone != OS timezone

Be careful with:

• Time zone

• Datetime format

• Fractional Seconds