25

In Python, I want to get the current Unix timestamp and then store the value for the long term and be handled by non-Python systems. (I am not merely trying to compute the difference between two timestamps within the same program run.)

Calling the function time.time() seems to be a very reasonable and concise way to get the desired timestamp... until I read the documentation:

Return the time in seconds since the epoch as a floating point number. The specific date of the epoch and the handling of leap seconds is platform dependent. On Windows and most Unix systems, the epoch is January 1, 1970, 00:00:00 (UTC) and leap seconds are not counted towards the time in seconds since the epoch. This is commonly referred to as Unix time. To find out what the epoch is on a given platform, look at gmtime(0).

[...]

(Versions: 3.5~3.9)

The phrase "epoch ... is platform dependent" is a warning sign. A weasel phrase is "most Unix systems". What are examples of Unix or non-Unix systems where Python's time.time()'s epoch is not 1970-01-01T00:00:00Z?

Is time.time() subtly unsafe for my goal? Should I look to alternatives like datetime.datetime.now().timestamp()?

Digging deeper, I also noticed that previous versions of Python didn't seem to have these caveats for time.time():

Return the time in seconds since the epoch as a floating point number. Note that even though the time is always returned as a floating point number, not all systems provide time with a better precision than 1 second. While this function normally returns non-decreasing values, it can return a lower value than a previous call if the system clock has been set back between the two calls.

(Versions: 2.7, 3.2~3.4)

And even older wording:

Return the time as a floating point number expressed in seconds since the epoch, in UTC. Note that even though the time is always returned as a floating point number, not all systems provide time with a better precision than 1 second. While this function normally returns non-decreasing values, it can return a lower value than a previous call if the system clock has been set back between the two calls.

(Versions: 2.2~2.6, 3.0~3.1)

Improve this question
7
  • 3
    I think any system conforming to POSIX must use that epoch. Commented Feb 26, 2021 at 22:54
  • As the documentation directs "To find out what the epoch is on a given platform, look at gmtime(0)" Commented Feb 27, 2021 at 15:24
  • 3
    side note: datetime.timestamp() falls back to time.time() - docs, src Commented Feb 28, 2021 at 16:24
  • You say you want to save a time value to be used by any system. The issue is what do you define as a system. Most desktop/server systems today are 64 bit machines, however, microprocessors are sometimes 32 bit machines. I believe, this potential difference is the rationale behind the statement that the epoch machine dependent. Commented Jan 9, 2023 at 22:25
  • 1
    What's up with the spike in activity this past month? Is it due to some software release where this issue became quite relevant? Commented Feb 14, 2023 at 5:43

1 Answer 1

Reset to default
7

If you don't want to depend upon the time.time() implementation and the (maybe) variable epoch, you can simply calculate the Unix timestamp yourself by getting the current datetime, and subtract the datetime of the epoch you want for the Unix timestamp (January 1st 1970), and get the seconds:

from datetime import datetime

unix_timestamp = (datetime.utcnow() - datetime(1970, 1, 1)).total_seconds()

NOTE: you might want to add the timezone information maybe.

EDIT: edited according to the comments, using datetime.utcnow seems the right way.

Improve this answer
Sign up to request clarification or add additional context in comments.

3 Comments

although mentioned already by you in the notes, I want to add the following unix_timestamp = (datetime.utcnow() - datetime(1970, 1, 1)).total_seconds() this will give you the correct time format, as the Unix Time is defined in UTC
You don't want timezone information and you shouldn't use now. You must use utcnow to get a proper unixtime. There is no such thing as local unixtime. You can use datetime.date.fromtimestamp to get local time which is a harder way to just use time.time to begin with
utcnow has since been deprecated and there's also the timestamp method. So you can just use datetime.now(timezone.utc).timestamp() or, if you only import the root datetime, datetime.datetime.now(datetime.UTC).timestamp().

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.