During development, I often found the concepts of timezone and format confusing.

So, I decided to write this to briefly organize them.

This post isn't about explaining how to use dayjs, but rather how dayjs can be applied within the concepts of time format and timezone.

1. Timezones: UTC vs GMT

1.1 GMT

GMT (Greenwich Mean Time) is the mean solar time measured at the Greenwich Observatory, used to set clocks.

It is 0 based on noon at the Greenwich Observatory.

It is not precise because it is determined based on the Earth's rotation. This is because the Earth's rotation is influenced by the movements of the Earth's core, oceans, and atmosphere.

Therefore, to reconcile the discrepancy between UTC and GMT (which will be explained later), leap seconds are applied to UTC to resolve the error.

1.2 UTC

UTC (Coordinated Universal Time) uses time measurement based on atomic clocks.

An atomic clock defines time by measuring the minute electronic transitions of cesium atoms.

You can find the principles of atomic clocks on YouTube, etc.

In any case, because of this, it is very accurate.

Similar to GMT, it is a timezone bounded by the Prime Meridian, but time measurement is based on atomic clocks.

GMT+1 and UTC+1 are, in other words, the same timezone.

2. Time Formats

2.1 ISO 8601

It was published in 1988 by the International Organization for Standardization.

It was developed to clearly define the representation of dates and times, preventing confusion in international contexts.

The YYYY.MM.DD format, commonly encountered in development, falls under this standard.

Notation

When connecting dates and times, the character T is used.

Timezones

Appending Z indicates UTC time.

2024-01-18T11:55:48.192831Z

To specify a local timezone instead of UTC, append +HH:MM.

2024-01-18T20:55:48.192831+09:00

2.2 Unix timestamp

It represents the time elapsed since January 1, 1970, 00:00:00 UTC, in seconds.

It is based solely on UTC time, which is not affected by timezones.

2.3 HTTP Date

https://developer.mozilla.org/ko/docs/Web/HTTP/Headers/Date

This is the date and time representation used in HTTP headers.

Sun, 06 Nov 1994 08:49:37 GMT

3. dayjs

Now that we've covered all the timezones and format types, let's apply them to dayjs!

dayjs().unix() // returns unix time 1705581000
dayjs().toString() // returns HTTP Date time "Thu, 18 Jan 2024 12:29:55 GMT"
dayjs().format() // returns time with UTC timezone "2024-01-18T21:31:17+09:00"
dayjs().ISOString() // returns UTC ISO time

It seems these are the only formats I've dealt with during development.

You can combine the above formats and apply them to your development.

For example, it would be applied in a way like this:

What if the server provides Unix time, but it needs to be displayed in ISO format?

Unix to ISO format

dayjs(unix * 1000).format()

Unix time is in seconds.

However, since JavaScript uses Unix time in milliseconds, we multiply by 1000 to convert it from seconds.

Conclusion

I've organized information related to timezones.

After organizing it once, the concepts seem to be much clearer.