UTC 시간과 파이썬 : 주의사항UTC Time and Python: Pitfalls

Checking the datetime output

Let’s look at the strings from before one more time. Since we went as far as passing a timezone in the code, you might think we followed the UTC format correctly. But the actual output differs from the ISO standard.

Sticking to the ISO format is very important. Not just in ordinary web front-end/back-end integration, but in any communication where you exchange data with other services, ignoring the standard can mean the two sides read different time values.

# Time strings produced by the code
2023-12-24 15:00:00.000000+00:00

2023-12-25 00:00:00.000000+09:00

Revisiting the ISO standard

Let’s recall the ISO standard notation once more. There are two main things to remember:

• the separator T between the date and the time
• Z or +00:00 to express the time zone offset

2023-12-24T15:00:00Z

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

Compared with the standard string, the code output is missing the separator T. So let’s see how to express a datetime object as an ISO-standard string.

1. strftime(): convert using a format string
2. isoformat(): convert to an ISO-standard string

strftime(): datetime object → string

The most basic approach is the strftime function. It converts a datetime object to a string using a given format string. As shown below, you can build a string that matches the ISO format to follow the standard, or build a string in whatever shape you like.

from datetime import datetime, timezone

utc_time = datetime.now(timezone.utc)
custom_str_time = utc_time.strftime("%Y/%m/%d")         # custom time string
iso_str_time = utc_time.strftime("%Y-%m-%dT%H:%M:%SZ")  # format following the ISO standard

print(utc_time)        # 2023-12-24 15:00+00:00
print(custom_str_time) # 2023/12/24
print(iso_str_time)    # 2023-12-24T15:00+00:00

Time format strings

There are many more formats beyond the basic ones below. See the official documentation.

The problem with expressing time zones

strftime seems to work just fine. The problem is when you have to express multiple time zones. In that case, you have to specify the right time zone string for each time zone yourself.

In the example below, a non-UTC time is labeled as UTC, which introduces a 9-hour discrepancy.

from datetime import datetime
from pytz import timezone

tz = timezone("Asia/Seoul")
now = datetime.now(tz)
iso_str_time = now.strftime("%Y-%m-%dT%H:%M:%SZ")  # UTC - format following the ISO standard

print(iso_str_time) # 2023-12-25T00:00+00:00 = expressed as a wrong UTC value

isoformat(): converting to ISO notation

With the strftime function above, you had to build the ISO-compliant string yourself. On top of that, you had to specify a separate string for each time zone.

The isoformat function makes this simple: it produces a standard-compliant string without you having to write it out by hand.

from datetime import datetime
from pytz import timezone

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

iso_str_time = now.isoformat()
iso_seconds_time = now.isoformat(sep="T", timespec="seconds")

print(now)               # 2023-12-25 00:00:00.0000+09:00
print(iso_str_time)      # 2023-12-25T00:00:00.000000+09:00 = includes the correct time zone
print(iso_seconds_time)  # 2023-12-25T00:00:00+09:00 = milliseconds removed

When calling isoformat, you can optionally pass two arguments.

Converting from strings

So far we’ve looked at converting a datetime object to a string.

Now let’s look at how to create a datetime object from a string.

1. strptime(): convert from a string that follows a given format
2. fromisoformat(): convert from an ISO-standard string

strptime(): string → datetime object

This is the most basic function for creating a time from a string. First, you build a format string that matches the format of the incoming time string. If the string doesn’t match the format, a ValueError is raised.

This function also has the problem of having to spell out each time zone. Taking the Seoul time zone as an example, you have to put exactly +09:00 in the format string to avoid an error.

from datetime import datetime

date_string = "2023-12-25T00:00:00+09:00"
date_format = "%Y-%m-%dT%H:%M:%S+09:00"

# convert to a datetime object
datetime_obj = datetime.strptime(date_string, date_format)

fromisoformat(): ISO format string → datetime object

If the string matches the ISO format, it creates a datetime object with the time zone applied for you. Even if the time separator is a space or some other character instead of T, it tries to parse the string accordingly: a single space or a single character other than T is handled automatically. Beyond that, however, parsing may fail if the format differs.

from datetime import datetime

date_string = "2023-12-25 00:00:00+09:00"
date_format = "%Y-%m-%dT%H:%M:%S+09:00"

# convert to a datetime object
datetime_obj = datetime.fromisoformat(date_string)
print(datetime_obj.tzinfo) # UTC+09:00

Converting to another time zone

This time, let’s see how to convert UTC to another time zone, or another time zone to UTC.

1. Calculating with a timedelta object
2. astimezone(): switching between time zones

Let’s say we want to convert a UTC time to Korean time, UTC+9, as below. (I’ll ignore milliseconds in this explanation.)

2023-12-24T15:00:00Z

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

Using a datedelta object

Korean time is 9 hours ahead of UTC, so you can simply add 9 hours. Here we use a datedelta object to add the 9 hours.

But when you check the result, there’s a problem. The time zone is still +00:00, meaning UTC. Contrary to what we intended, it now represents a UTC time 9 hours ahead of the original.

from datetime import datetime, timezone, timedelta

utc_time = datetime.now(timezone.utc)
kor_time = utc_time + timedelta(hours=9)

print(utc_time) # 2023-12-24 15:00:00+00:00
print(kor_time) # 2023-12-25 00:00:00+00:00
print(kor_time.tzinfo) # UTC

astimezone(): UTC ↔ other time zones

The astimezone function makes it easy to switch between time zones. It takes the time zone information into account when converting.

You need to specify the target time zone precisely. For example, it’s better to use exact time zone names such as ‘America/New_York’ or ‘Asia/Seoul’ rather than abbreviations like ‘EST’ or ‘CST’. Use the pytz library to set the exact time zone.

from datetime import datetime, UTC
from pytz import timezone

utc_time = datetime.now(UTC)
target_timezone = timezone("Asia/Seoul")

kor_time = utc_time.astimezone(target_timezone)

print(utc_time)        # 2023-12-24 15:00:00+00:00
print(kor_time)        # 2023-12-25 00:00:00+09:00
print(kor_time.tzinfo) # Asia/Seoul

Wrapping up

When exchanging data using UTC time, you must use time zone strings that follow the standard to avoid confusion. And when calculating times, check the time zone of each time and use the right function for it so that there are no errors caused by time zones.

1. Use isoformat() rather than strftime()
2. Use astimezone() rather than calculating directly with a timedelta object

That covers the basics of handling UTC in Python. Thanks for reading! 😊