Python: using time functions

July 17, 2020

Back-End

How to use local and UTC time in python

Reading UTC (Coordinated Universal Time aka Greenwich Mean Time - GMT):

• as seconds since the Epoch - 1/1/1970:

>>> import time
>>> time.time()
1464775923.631278

• as struct_time structure:

>>> time.gmtime()
time.struct_time(tm_year=2016, tm_mon=6, tm_mday=1, tm_hour=10, tm_min=35, tm_sec=39, tm_wday=2, tm_yday=153, tm_isdst=0)

• as datetime structure:

>>> import datetime
>>> datetime.datetime.now()
datetime.datetime(2016, 6, 1, 13, 38, 53, 477147)

Our absolute reference system time is UTC (GMT).
In order to know local time, we have to make adjustemens (DST = Daylight Saving Time) relative to UTC time.
Time structures could be aware of these adjustements (include utc + local offset adjustement (tzinfo)) or not (in this case we have "naive date").

Setting system local time on raspbian:

• Running sudo raspi-config : Internalization options / Change Timezone
• Manually: sudo cp /usr/share/zoneinfo/Europe/Bucharest /etc/localtime

Reading UTC time as struct_time (no offset info):

>>> time.gmtime()
time.struct_time(tm_year=2016, tm_mon=6, tm_mday=1, tm_hour=11, tm_min=13, tm_sec=49, tm_wday=2, tm_yday=153, tm_isdst=0)

Reading local time as struct_time:

>>> time.localtime()
time.struct_time(tm_year=2016, tm_mon=6, tm_mday=1, tm_hour=14, tm_min=14, tm_sec=1, tm_wday=2, tm_yday=153, tm_isdst=1)

Reading UTC time as naive datetime:

>>> utc = datetime.datetime.utcnow()
>>> print utc, "tzinfo =", utc.tzinfo
2016-06-01 12:15:45.431271 tzinfo = None
>>> utc.timetuple()
time.struct_time(tm_year=2016, tm_mon=6, tm_mday=1, tm_hour=12, tm_min=15, tm_sec=45, tm_wday=2, tm_yday=153, tm_isdst=-1)

Reading local time as naive datetime:

>>> local = datetime.datetime.now()
>>> print local, "tzinfo =", local.tzinfo
2016-06-01 15:17:27.293524 tzinfo = None
>>> local.timetuple()
time.struct_time(tm_year=2016, tm_mon=6, tm_mday=1, tm_hour=15, tm_min=17, tm_sec=27, tm_wday=2, tm_yday=153, tm_isdst=-1)

Reading UTC time as aware datetime (offset = 0):

>>> import pytz
>>> utc = datetime.datetime.now(tz=pytz.utc)
>>> print utc, "tzinfo =", utc.tzinfo
2016-06-01 12:18:41.062542+00:00 tzinfo = UTC
>>> utc.timetuple()
time.struct_time(tm_year=2016, tm_mon=6, tm_mday=1, tm_hour=12, tm_min=18, tm_sec=41, tm_wday=2, tm_yday=153, tm_isdst=0)

Reading local time as aware datetime:

>>> bucharest = pytz.timezone('Europe/Bucharest')
>>> local = datetime.datetime.now(tz=bucharest)
>>> print local, "tzinfo =", local.tzinfo
2016-06-01 15:20:18.439973+03:00 tzinfo = Europe/Bucharest
>>> local.timetuple()
time.struct_time(tm_year=2016, tm_mon=6, tm_mday=1, tm_hour=15, tm_min=20, tm_sec=18, tm_wday=2, tm_yday=153, tm_isdst=1)

Conversions time_struct to/from epochseconds and formatted output:

mport time, calendar

utc = time.gmtime()
print "utc time_struct: ", utc

print "formatted output1: ", time.strftime("%a, %d %b %Y %H:%M:%S  %Z", utc)
print "formatted output2: ", time.asctime(utc)

epochseconds = calendar.timegm(utc)
print "epochseconds: %s" % (epochseconds)
print "utc time_struct: ", time.gmtime(epochseconds)

print

local = time.localtime()
print "local time_struct: ", local

print "formatted output1: ", time.strftime("%a, %d %b %Y %H:%M:%S  %Z", local)
print "formatted output2: ", time.asctime(local)

epochseconds = time.mktime(local)
print "epochseconds: %s" % (epochseconds)
print "local time_struct: ", time.localtime(epochseconds)

result:

utc time_struct:  time.struct_time(tm_year=2016, tm_mon=6, tm_mday=2, tm_hour=13, tm_min=48, tm_sec=34, tm_wday=3, tm_yday=154, tm_isdst=0)
formatted output1:  Thu, 02 Jun 2016 13:48:34  EET
formatted output2:  Thu Jun  2 13:48:34 2016
epochseconds: 1464875314
utc time_struct:  time.struct_time(tm_year=2016, tm_mon=6, tm_mday=2, tm_hour=13, tm_min=48, tm_sec=34, tm_wday=3, tm_yday=154, tm_isdst=0)
local time_struct:  time.struct_time(tm_year=2016, tm_mon=6, tm_mday=2, tm_hour=16, tm_min=48, tm_sec=34, tm_wday=3, tm_yday=154, tm_isdst=1)
formatted output1:  Thu, 02 Jun 2016 16:48:34  EEST
formatted output2:  Thu Jun  2 16:48:34 2016
epochseconds: 1464875314.0
local time_struct:  time.struct_time(tm_year=2016, tm_mon=6, tm_mday=2, tm_hour=16, tm_min=48, tm_sec=34, tm_wday=3, tm_yday=154, tm_isdst=1)

Reading different local times as time_struct:

import os, time

try:
    print os.environ['TZ']
except:
    print "TZ is not defined, using system local settings"

print "Local system time - timezone (%s), tzname (%s)" % (time.timezone, time.tzname)
print time.strftime('%X %x %Z')
print time.localtime()

print "US/Eastern - timezone (%s), tzname (%s)" % (time.timezone, time.tzname)
os.environ['TZ'] = 'US/Eastern'
time.tzset()
print time.strftime('%X %x %Z')
print time.localtime()

print "Australia/Melbourne - timezone (%s), tzname (%s)" % (time.timezone, time.tzname)
os.environ['TZ'] = 'Australia/Melbourne'
time.tzset()
print time.strftime('%X %x %Z')
print time.localtime()

print "Europe/Bucharest - timezone (%s), tzname (%s)" % (time.timezone, time.tzname)
os.environ['TZ'] = "Europe/Bucharest"
time.tzset()
print time.strftime('%X %x %Z')
print time.localtime()

result:

TZ is not defined, using system local settings
Local system time - timezone (-7200), tzname (('EET', 'EEST'))
11:47:13 06/02/16 EEST
time.struct_time(tm_year=2016, tm_mon=6, tm_mday=2, tm_hour=11, tm_min=47, tm_sec=13, tm_wday=3, tm_yday=154, tm_isdst=1)
US/Eastern - timezone (-7200), tzname (('EET', 'EEST'))
04:47:13 06/02/16 EDT
time.struct_time(tm_year=2016, tm_mon=6, tm_mday=2, tm_hour=4, tm_min=47, tm_sec=13, tm_wday=3, tm_yday=154, tm_isdst=1)
Australia/Melbourne - timezone (18000), tzname (('EST', 'EDT'))
18:47:13 06/02/16 AEST
time.struct_time(tm_year=2016, tm_mon=6, tm_mday=2, tm_hour=18, tm_min=47, tm_sec=13, tm_wday=3, tm_yday=154, tm_isdst=0)
Europe/Bucharest - timezone (-36000), tzname (('AEST', 'AEDT'))
11:47:13 06/02/16 EEST
time.struct_time(tm_year=2016, tm_mon=6, tm_mday=2, tm_hour=11, tm_min=47, tm_sec=13, tm_wday=3, tm_yday=154, tm_isdst=1)
08:47:13 06/02/16 UTC
time.struct_time(tm_year=2016, tm_mon=6, tm_mday=2, tm_hour=8, tm_min=47, tm_sec=13, tm_wday=3, tm_yday=154, tm_isdst=0)

Converting date/time values between different zones

The preferred way of dealing with times is to always work in UTC, converting to localtime only when generating output to be read by humans.

import time, os, datetime, pytz
bucharest = pytz.timezone('Europe/Bucharest')

#Using astimezone

year1           = 2016
month1          = 6
day1            = 2
hour1           = 17
min1            = 0
sec1            = 0
microsec1       = 0

utc = datetime.datetime(year1, month1, day1, hour1, min1, sec1, microsec1, tzinfo=pytz.utc)
local = utc.astimezone(bucharest)

print utc, utc.tzinfo
print local, local.strftime('%z'), local.utcoffset(), local.dst(), local.tzinfo
print local - utc

year2            = 2016
month2           = 12
day2             = 2
hour2            = 17
min2             = 0
sec2             = 0
microsec2        = 0

utc = datetime.datetime(year2, month2, day2, hour2, min2, sec2, microsec2, tzinfo=pytz.utc)
local = utc.astimezone(bucharest)

print utc, utc.tzinfo
print local, local.strftime('%z'), local.utcoffset(), local.dst(), local.tzinfo
print local - utc


#Using pytz.localize

#naive dates
date1 = datetime.date(2016,6,2)
date2 = datetime.date(2016,12,2)
time = datetime.time(17,0,0)
dt1 = datetime.datetime.combine(date1,time)
dt2 = datetime.datetime.combine(date2,time)

#aware localized dates
local1 = pytz.timezone('Europe/Bucharest').localize(dt1)
local2 = pytz.timezone('Europe/Bucharest').localize(dt2)
print local1, local1.strftime('%z'), local1.utcoffset(), local1.dst(), local1.tzinfo
print local2, local2.strftime('%z'), local2.utcoffset(), local2.dst(), local2.tzinfo

result:

2016-06-02 17:00:00+00:00 UTC
2016-06-02 20:00:00+03:00 +0300 3:00:00 1:00:00 Europe/Bucharest
0:00:00
2016-12-02 17:00:00+00:00 UTC
2016-12-02 19:00:00+02:00 +0200 2:00:00 0:00:00 Europe/Bucharest
0:00:00
2016-06-02 17:00:00+03:00 +0300 3:00:00 1:00:00 Europe/Bucharest
2016-12-02 17:00:00+02:00 +0200 2:00:00 0:00:00 Europe/Bucharest

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