Links

• The tz database: Time Zone and Daylight Saving Time
• Libraries
  • pytz  (pypi)
  • python-dateutil

Parsing

• Python strptime() and timezones?
• tuple_time = time.strptime(date_string, "%Y%m%dT%H:%M:%S")
  • See 16.3. time — Time access and conversions

Formatting

• datetime_obj.ctime() gives you the standard Thu Jun 20 00:00 2009 style.
• datetime_obj.strftime(fmt)
  • Look up fmt here - strftime() and strptime() Behavior

time.time()

• Note:  To measure wall clock time, prefer timeit.default_timer() instead of time.time()
• 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.

time.ctime([secs])

• Convert a time expressed in seconds since the epoch to a string representing local time. If secs is not provided or None, the current time as returned by time() is used. ctime(secs) is equivalent to asctime(localtime(secs)). Locale information is not used by ctime().

Epoch Time

• To epoch time from datetime

  time.mktime(date_time_obj.timetuple())
  

• If you have time in epoch time (i.e. a number), then convert it to datetime like this.

  # epoch_secs is in epoch time  
  datetime.datetime(*time.localtime(epoch_secs)[:6])
  

• The time value as returned by gmtime(), localtime() and strptime() and accepted by asctime(), mktime() and strftime() may be considered as a sequence of 9 integers. The return values of gmtime(), localtime() and strptime() also offer attribute names for individual fields.

  Index	Attribute	Values
  0	tm_year	(for example, 1993)
  1	tm_mon	range [1,12]
  2	tm_mday	range [1,31]
  3	tm_hour	range [0,23]
  4	tm_min	range [0,59]
  5	tm_sec	range [0,61]; see (1) in strftime()
  6	tm_wday	range [0,6], Monday is 0
  7	tm_yday	range [1,366]
  8	tm_isdst	0, 1 or -1; see below

  Note that unlike the C structure, the month value is a range of 1-12, not 0-11. A year value will be handled as described under “Year 2000 (Y2K) issues” above. A -1 argument as the daylight savings flag, passed to mktime() will usually result in the correct daylight savings state to be filled in.

When a tuple with an incorrect length is passed to a function expecting a struct_time, or having elements of the wrong type, a TypeError is raised.

• Use the following functions to convert between time representations:

  From	To	Use
  seconds since epoch in UTC	struct_time	gmtime()
  seconds since epoch in local	struct_time	localtime()
  struct_time	seconds since epoch in UTC	calendar.timegm()
  struct_time	seconds since epoch in time	mktime()

Timestamps

import datetime, calendar

# Get current UTC timestamp
def timestamp_utcnow():
  utc_dt = datetime.datetime.utcnow()
  return int(calendar.timegm(utc_dt.utctimetuple()))

Translating between timezones

Translate to Eastern Timezone

import calendar
import datetime
import pytz
import time

now = datetime.datetime.now()
# First, translate to UTC.
now = datetime.datetime.utcfromtimestamp(time.mktime(now.timetuple()))
# Next, stick in the tzinfo to mark it as UTC (pytz requires it.)
now = now.replace(tzinfo=pytz.timezone("UTC"))
# Finally, translate to the target timezone.
now = now.astimezone(pytz.timezone("US/Eastern"))

def parse_utc_timestamp(utc_timestamp):
  date_obj = datetime.datetime.utcfromtimestamp(utc_timestamp)
  # This is needed for any .astimezone() calls done later.
  date_obj = date_obj.replace(tzinfo=pytz.timezone("UTC"))
  return date_obj

def local_to_utc(local_date_obj):
  # Translate to UTC.  We use time.mktime().
  # The parallel function, calendar.timegm(), takes it's
  # input in UTC and not local.
  utc_timestamp = time.mktime(local_date_obj.timetuple())
  return parse_utc_timestamp(utc_timestamp)

def utc_to_eastern(date_obj):
  date_obj = date_obj.astimezone(pytz.timezone("US/Eastern"))
  return date_obj

def utc_to_pacific(date_obj):
  date_obj = date_obj.astimezone(pytz.timezone("US/Pacific"))
  return date_obj


print "Eastern stuff"

now = datetime.datetime.now()
print "local =", now
now_utc = local_to_utc(now)
print "local -> utc =", now_utc
print "utcnow =", datetime.datetime.utcnow()

now_eastern = utc_to_eastern(now_utc)
print now_eastern