1

I'm trying to follow these answers and get the elapsed seconds since Jan 1st 1970 for each element in my array (my_times). I'm then using these values to find the time intervals between each consecutive time. Either approach I take seems to give the wrong answer for at least one pair of times.

Mark Byers answer

To get the seconds since Jan 1st 1970, he suggested to try:

time.mktime(my_time.timetuple())

However this does not seem to work for times "2017-11-05 01:46:00+00" and "2017-11-05 01:47:00+00".

When I run the below code, it says the values are separated by 3660.0 seconds instead of 60.0 seconds

from datetime import datetime
import time

my_time1 = datetime.strptime("2017-11-05 01:46:00+00", "%Y-%m-%d %H:%M:%S+%f")
my_time2 = datetime.strptime("2017-11-05 01:47:00+00", "%Y-%m-%d %H:%M:%S+%f")
time.mktime(my_time2.timetuple()) - time.mktime(my_time1.timetuple())

Andrzej Pronobis' answer

To get the seconds since Jan 1st 1970, he suggested to try:

my_time.timestamp()

This fixed the two earlier times however it no longer works for times "2017-11-05 01:59:00+00" and "2017-11-05 02:00:00+00". The same issue appears, I get 3660.0 seconds instead of 60.0 seconds

from datetime import datetime

my_time1 = datetime.strptime("2017-11-05 01:59:00+00", "%Y-%m-%d %H:%M:%S+%f")
my_time2 = datetime.strptime("2017-11-05 02:00:00+00", "%Y-%m-%d %H:%M:%S+%f")
my_time2.timestamp() - my_time1.timestamp()

I'd like to know if I'm doing anything wrong? Also is there a better way to find all consecutive time intervals when the datetime is given as a String?

Edit:

Thank you John, that fixed the problem. Oddly, changing the format from +%f to %z still ran into the same issue.

What did work was running sudo ln -sf /usr/share/zoneinfo/UTC /etc/localtime (changes my computer's time to UTC) and then evaluating all the times

Patrick Stetz
  • 455
  • 2
  • 7
  • 14
  • 1
    Your code works for me and it seems strange that 01:46 and 01:47 should differ. If you just need the difference between two datetime objs you could use: (my_time2 - my_time1).total_seconds() – Anton vBR Apr 01 '18 at 02:13
  • 5
    The error of 3600 seconds is equal to 1 hour (60*60 seconds). This is most likely an issue to do with daylight savings time. Convert your times to UTC and then subtract – Oscar Benjamin Apr 01 '18 at 02:14

1 Answers1

3

This is a case of "garbage in, garbage out." Here:

datetime.strptime("2017-11-05 01:59:00+00", "%Y-%m-%d %H:%M:%S+%f")

You probably think that +00 on the end means "UTC time", but the %f format specifier means "fractional seconds."

In any case, you're apparently running on a system where the time zone is set to one with daylight saving time part of the year. 2 AM happens twice on the DST changeover date in November, so your code is working as written (it's ambiguous, basically).

Put another way: your issue is not that you're computing time deltas incorrectly. Your issue is that you are loading the times from strings incorrectly (or ambiguously).

John Zwinck
  • 239,568
  • 38
  • 324
  • 436