UTC 시간과 파이썬 : 기초UTC Time and Python: The Basics

What are GMT and UTC?

GMT (Greenwich Mean Time) is the standard time derived from measuring the position of the sun at the Royal Observatory in Greenwich, London.

UTC (Coordinated Universal Time) is based on GMT, but it is adjusted much more precisely using atomic clocks. It has been the international time standard since January 1, 1972, and it counts elapsed time in milliseconds, taking midnight on January 1, 1970 as 0 ms. This accumulated time is called a timestamp.

UTC plays a key role in computer systems and networking for things like recording data, managing logs, and synchronizing clocks. As the snippets below show, you can see UTC at work in many programming languages with just a few lines of code.

const Jan01_1970 = new Date(0);
Jan01_1970.valueOf() // 0

const now = new Date();
now.valueOf()        // 170........ milliseconds elapsed since 1970-01-01

from datetime import datetime

now = datetime.now()
print(now.timestamp()) # 170........ milliseconds elapsed since 1970-01-01

ISO 8601

So how should UTC be written? To express dates, times, and time zones in a clear, consistent format, you follow the ISO 8601 standard. Under ISO 8601, a UTC time can be written like this:

2023-12-24T15:00:00Z

2023-12-24T15:00:00+00:00

• The T in the middle is the separator between the date and the time.
• The Z at the end stands for the Zero time zone, i.e. UTC.
• Expressed as a time zone offset, Z is +00:00.

Time zone offsets

Many countries and regions have their own standard time zone, defined as an offset from UTC. For example, Korea Standard Time (KST) is UTC+9, and New York (Eastern Standard Time, EST) is UTC-5.

Local time is calculated by adding or subtracting the region’s offset from UTC. For example, when it is Christmas Eve in the UK (offset 0, i.e. UTC) = 2023-12-24 at 15:00, it is Christmas Day in Korea (offset +09:00) = 2023-12-25 at 00:00.

2023-12-24T15:00:00Z

2023-12-25T00:00:00+09:00

UTC in Python

These are the main datetime module functions for working with UTC time in Python:

• datetime.now()
• datetime.utcnow()
• the pytz library

datetime.now()

Returns the current local system time. Called with no arguments, it returns a naive datetime object with no time zone information. In other words, a datetime created with datetime.now() doesn’t know its time zone, so you can’t explicitly compare it with or convert it to a concrete time zone such as UTC. You can check the time zone information through the tzinfo attribute.

from datetime import datetime

now = datetime.now()

print(now)        # 2023-12-25 00:00:00.000000
print(now.tzinfo) # None

datetime.utcnow()

This function returns the current UTC time. However, it also returns a naive datetime object with no time zone information, which makes it hard to use UTC properly. On top of that, it has been deprecated since Python 3.12, so I recommend not using it.

from datetime import datetime, UTC

utc_now = datetime.utcnow()

print(utc_now)        # 2023-12-24 15:00:00.000000
print(utc_now.tzinfo) # None

Aware and naive objects

Date and time objects can be classified as aware or naive, depending on whether they include time zone information.

Aware

An aware object can locate itself relative to other aware objects. This also makes it possible to apply daylight saving time (DST, a.k.a. summer time). In short, it lets you use and calculate time unambiguously.

Naive

A naive object doesn’t carry enough information to locate itself relative to other times. Whether a naive object represents UTC, local time, or some other time zone is entirely up to the program. Naive objects are easy to understand and easy to work with, but they can cause problems in time-handling logic.

Adding a timezone

So how do you create a time object with the correct UTC offset applied? Just pass a timezone object, which represents an offset, when you create the time. You can use timezone.utc, which means UTC 0, or, in Python 3.12, the newly added UTC constant.

from datetime import datetime, timezone, UTC

now = datetime.now(timezone.utc)

print(now)        # 2023-12-24 15:00:00.000000+00:00
print(now.tzinfo) # UTC

Setting the +09:00 Korean offset

For time zones other than UTC+00:00, create a timezone object yourself using a timedelta object for the offset. You can see that the output now ends with the +09:00 time zone offset, and that the tzinfo attribute is set.

from datetime import datetime, timezone, timedelta

tz = timezone(timedelta(hours=9))
now = datetime.now(tz)

print(now)        # 2023-12-25 00:00:00.000000+09:00
print(now.tzinfo) # UTC+09:00

The pytz library

Imagine building a timezone for every country by hand from its offset. You would have to know which offset each city in each country uses. Instead, use the pytz library, which provides information on more than 500 time zones, and you can easily work with many time zones, including UTC.

$ pip install pytz

from datetime import datetime
from pytz import timezone, all_timezones

tz = timezone("Asia/Tokyo")
now = datetime.now(tz)

print(now)           # 2023-12-25 00:00:00.000000+09:00
print(now.tzinfo)    # Asia/Tokyo
print(all_timezones) # lists the supported time zone strings

Wrapping up

UTC is the international time standard, and many systems and programs use it. To write UTC correctly, follow ISO 8601. But take another look at the output above. The code we’ve seen has a catch. Look closely one more time.

Really? Is it actually following the ISO standard…

# Times produced by the code
2023-12-24 15:00:00.000000+00:00
2023-12-25 00:00:00.000000+09:00

# ISO standard
2023-12-24T15:00:00.000000+00:00
2023-12-25T00:00:00.000000+09:00

You can see that the T separator between the date and the time is missing. Small deviations from the standard like this can lead to a few problems. 🥲

What’s next

In UTC Time and Python: Pitfalls, we’ll look at the problems I actually ran into while implementing this at work.

1. The date-time separator
2. How time zones are expressed
3. Calculating with time zone offsets in mind