Date and Time

Dates and Times

First, a quick review of time and clocks wrt GNU/Linux:

• Epoch time is the number of seconds since 1970-01-01 00:00:00 UTC. Aka “timestamp” or “unix time”.
• DST (daylight savings time) adjustments are only made to local time, not to UTC. So, your local time may be a differing number of hours off from UTC depending upon what time of year it is (that is, whether or not DST is in effect).
• The hwclock (BIOS), aka RTC (real time clock) is separate from the system (OS) clock. Regular GNU/Linux machines have their hardware clock set to UTC.
• When you boot up, your OS system clock gets its time from the hwclock, and your OS knows your timezone and whether or not DST is in effect.
• You need root access to run the hwclock command. Note that by default hwclock shows h/w clock time in local time, even though it’s set to UTC.

To see more details of what your h/w and system clocks are set to:

$ timedatectl      # Shows various info about your clocks.
# hwclock --debug  # More info.
$ date +%s   # unix/posix/epoch time
$ date       # human-readable, local
$ date -u    # human-readable, UTC

The upshot of all this is: store your times in epoch time. You can always convert them later to a local time for displaying them.

Python Dates and Times

The time module

Python uses a handful of different time formats, including:

• seconds since epoch (a float)
• “time-tuple” (tuple of 9 integers)
• human-readable date/time string

A time-tuple has fields:

• tm_year (integer year)
• tm_mon (1–12)
• tm_mday (1–31)
• tm_hour (0–23)
• tm_min (0–59)
• tm_sec (0–61, to cover leap-seconds)
• tm_wday (0–6, Mon == 0)
• tm_yday (1–366)
• tm_isdst (-1 == library determines dst, 0 == no, 1 == yes)

Recall that DST is on (== 1) after you spring ahead and until you fall back.

To convert between Python’s basic time formats, see these functions in the time module:

Ex:

import time
time.strptime('2018-04-16', '%Y-%m-%d')  #=> corresponding time-tuple

Sleep

import time
time.sleep(3)  # 3 seconds

The datetime module

The main classes in the datetime module are: date, time, datetime, timedelta, and timezone. Times and datetimes can be naive (of tz info) or aware. Dates are always naive.

See the docs for datetime.