Sudden jump in time while using time.local() in Python

Question

I am trying to create a stopwatch using python. I have used time module for my purpose. But while executing the the following code I am getting some weird sudden jump in time. And one thing I noticed is that the jump is always of 41 seconds. Output image showing jump in Time elapsed = 20s Why does this happen and how do I avoid it?

import time
t = time.localtime()
ct = int(time.strftime('%H%M%S',t))
while True:
    t = time.localtime()
    ct2 = int(time.strftime('%H%M%S',t))
    time_elapsed = ct2 - ct
    #print(ct2,ct)
    h = '%02d' %(time_elapsed//3600)
    m = '%02d' %(time_elapsed//60)
    s = '%02d' %(time_elapsed-3600*int(h)-60*int(m))
    print('Time elapsed: {}:{}:{}'.format(h,m,s))
    time.sleep(1)

Code Output:

Time elapsed: 00:00:01
Time elapsed: 00:00:02
Time elapsed: 00:00:03
Time elapsed: 00:00:04
Time elapsed: 00:00:05
Time elapsed: 00:00:06
Time elapsed: 00:00:07
Time elapsed: 00:00:08
Time elapsed: 00:00:09
Time elapsed: 00:00:10
Time elapsed: 00:00:11
Time elapsed: 00:00:12
Time elapsed: 00:00:13
Time elapsed: 00:00:14
Time elapsed: 00:00:15
Time elapsed: 00:00:16
Time elapsed: 00:00:17
Time elapsed: 00:00:18
Time elapsed: 00:00:19
Time elapsed: 00:00:20
Time elapsed: 00:01:01
Time elapsed: 00:01:02
Time elapsed: 00:01:03
Time elapsed: 00:01:04
Time elapsed: 00:01:05
Time elapsed: 00:01:06

the conversion of an h-m-s string to integer to calculate durations seems complicated and error-prone. Why not do the calculation in seconds and then [format to string](https://stackoverflow.com/questions/538666/format-timedelta-to-string)? Why not use a [timedelta](https://docs.python.org/3/library/datetime.html#timedelta-objects)? — FObersteiner, May 31 '21 at 17:17

score 2 · Answer 1 · answered May 31 '21 at 18:07

This happens each time c2 increments the 100's digit. I uncommented print(c2, ct) to demonstrate:

100057 100031
Time elapsed: 00:00:26
100058 100031
Time elapsed: 00:00:27
100059 100031
Time elapsed: 00:00:28
100100 100031
Time elapsed: 00:01:09
100101 100031
Time elapsed: 00:01:10

Why

Note the jump in ct2 from 100059 to 100100. This happens because the code converts a str to an int: ct2 = int(time.strftime('%H%M%S',t)). The str is actually a base24 digit followed by two base60 digits and should never be converted to an integer. This only works because t isn't in the 0th hour of the day, e.g. 00:00:10.

Solution

A solution is to get the time as UNIX time, perform the arithmetic, and convert it to the format you want.

import time
start_time = time.time()
while True:
    current_time = time.time()
    time_elapsed = current_time - start_time
    print(f'Time elapsed: {time.strftime("%H:%M:%S", time.gmtime(time_elapsed))}')
    time.sleep(1)

Now, this solution will fail if more than 86400s (one day) has elapsed. For example: when exactly 25 hours have elapsed, this solution will print 01:00:00.

Another solution is to use builtin date/time arithmetic, which returns a datetime.timedelta.

from datetime import datetime, timedelta
import time
start_time = datetime.now()
while True:
    current_time = datetime.now()
    time_elapsed = timedelta(current_time - start_time)
    print(f'Time elapsed: {time_elapsed}')
    time.sleep(1)

Note that this solution outputs a slightly different format for elapsed time under 24 hours, afterwards its output looks like this:

1 day, 16:00:10

Advice

Stop trying to do your own date/time arithmetic. It's much more difficult than most people realize, and a constant source of material for The Daily WTF. Anytime a programmer tries to mangle dates or times on their own I point them at this list (content reproduced below).

Falsehoods programmers believe about time

This is a compiled list of falsehoods programmers tend to believe about working with time.

Don't re-invent a date time library yourself. If you think you understand everything about time, you're probably doing it wrong.

Falsehoods

There are always 24 hours in a day.
February is always 28 days long.
Any 24-hour period will always begin and end in the same day (or week, or month).
A week always begins and ends in the same month.
A week (or a month) always begins and ends in the same year.
The machine that a program runs on will always be in the GMT time zone.
Ok, that’s not true. But at least the time zone in which a program has to run will never change.
Well, surely there will never be a change to the time zone in which a program hast to run in production.
The system clock will always be set to the correct local time.
The system clock will always be set to a time that is not wildly different from the correct local time.
If the system clock is incorrect, it will at least always be off by a consistent number of seconds.
The server clock and the client clock will always be set to the same time.
The server clock and the client clock will always be set to around the same time.
Ok, but the time on the server clock and time on the client clock would never be different by a matter of decades.
If the server clock and the client clock are not in synch, they will at least always be out of synch by a consistent number of seconds.
The server clock and the client clock will use the same time zone.
The system clock will never be set to a time that is in the distant past or the far future.
Time has no beginning and no end.
One minute on the system clock has exactly the same duration as one minute on any other clock
Ok, but the duration of one minute on the system clock will be pretty close to the duration of one minute on most other clocks.
Fine, but the duration of one minute on the system clock would never be more than an hour.
The smallest unit of time is one second.
Ok, one millisecond.
It will never be necessary to set the system time to any value other than the correct local time.
Ok, testing might require setting the system time to a value other than the correct local time but it will never be necessary to do so in production.
Time stamps will always be specified in a commonly-understood format like 1339972628 or 133997262837.
Time stamps will always be specified in the same format.
Time stamps will always have the same level of precision.
A time stamp of sufficient precision can safely be considered unique.
A timestamp represents the time that an event actually occurred.
Human-readable dates can be specified in universally understood formats such as 05/07/11.
The offsets between two time zones will remain constant.
OK, historical oddities aside, the offsets between two time zones won’t change in the future.
Changes in the offsets between time zones will occur with plenty of advance notice.
Daylight saving time happens at the same time every year.
Daylight saving time happens at the same time in every time zone.
Daylight saving time always adjusts by an hour.
Months have either 28, 29, 30, or 31 days.
The day of the month always advances contiguously from N to either N+1 or 1, with no discontinuities.
There is only one calendar system in use at one time.
There is a leap year every year divisible by 4.
Non leap years will never contain a leap day.
It will be easy to calculate the duration of x number of hours and minutes from a particular point in time.
The same month has the same number of days in it everywhere!
Unix time is completely ignorant about anything except seconds.
Unix time is the number of seconds since Jan 1st 1970.
The day before Saturday is always Friday.
Contiguous timezones are no more than an hour apart. (aka we don’t need to test what happens to the avionics when you fly over the International Date Line)
Two timezones that differ will differ by an integer number of half hours.
Okay, quarter hours.
Okay, seconds, but it will be a consistent difference if we ignore DST.
If you create two date objects right beside each other, they’ll represent the same time. (a fantastic Heisenbug generator)
You can wait for the clock to reach exactly HH:MM:SS by sampling once a second.
If a process runs for n seconds and then terminates, approximately n seconds will have elapsed on the system clock at the time of termination.
Weeks start on Monday.
Days begin in the morning.
Holidays span an integer number of whole days.
The weekend consists of Saturday and Sunday.
It’s possible to establish a total ordering on timestamps that is useful outside your system.
The local time offset (from UTC) will not change during office hours.
Thread.sleep(1000) sleeps for 1000 milliseconds.
Thread.sleep(1000) sleeps for >= 1000 milliseconds.
There are 60 seconds in every minute.
Timestamps always advance monotonically.
GMT and UTC are the same timezone.
Britain uses GMT.
Time always goes forwards.
The difference between the current time and one week from the current time is always 7 * 86400 seconds.
The difference between two timestamps is an accurate measure of the time that elapsed between them.
24:12:34 is a invalid time.
Every integer is a theoretical possible year.
If you display a datetime, the displayed time has the same second part as the stored time,
Or the same year,
But at least the numerical difference between the displayed and stored year will be less than 2.
If you have a date in a correct YYYY-MM-DD format, the year consists of four characters.
If you merge two dates, by taking the month from the first and the day/year from the second, you get a valid date.
But it will work, if both years are leap years
If you take a w3c published algorithm for adding durations to dates, it will work in all cases.
The standard library supports negative years and years above 10000.
Time zones always differ by a whole hour.
If you convert a timestamp with millisecond precision to a date time with second precision, you can safely ignore the millisecond fractions.
But you can ignore the millisecond fraction, if it is less than 0.5.
Two-digit years should be somewhere in the range 1900-2099.
If you parse a date time, you can read the numbers character for character, without needing to backtrack.
But if you print a date time, you can write the numbers character for character, without needing to backtrack.
You will never have to parse a format like ---12Z or P12Y34M56DT78H90M12.345S.
There are only 24 time zones.
Time zones are always whole hours away from UTC.
Daylight Saving Time (DST) starts/ends on the same date everywhere.
DST is always an advancement by 1 hour.
Reading the client’s clock and comparing to UTC is a good way to determine their timezone.
The software stack will/won’t try to automatically adjust for timezone/DST.
My software is only used internally/locally, so I don’t have to worry about timezones.
My software stack will handle it without me needing to do anything special.
I can easily maintain a timezone list myself.
All measurements of time on a given clock will occur within the same frame of reference.
The fact that a date-based function works now means it will work on any date.
Years have 365 or 366 days.
Each calendar date is followed by the next in sequence, without skipping.
A given date and/or time unambiguously identifies a unique moment.
Leap years occur every 4 years.
You can determine the time zone from the state/province.
You can determine the time zone from the city/town.
Time passes at the same speed on top of a mountain and at the bottom of a valley.
One hour is as long as the next in all time systems.
You can calculate when leap seconds will be added.
The precision of the data type returned by a getCurrentTime() function is the same as the precision of that function.
Two subsequent calls to a getCurrentTime() function will return distinct results.
The second of two subsequent calls to a getCurrentTime() function will return a larger result.
The software will never run on a space ship that is orbiting a black hole.
Devices will be set to the local timezone
Users prefer to use the local timezone

Sources

This list is based on these articles. More detailed information about each statement can be found in one of two articles.