outline.txt

# Theory: Dates and times for humans

  - Times are pretty straightforward
    Cut up the day into regular intervals
    will never change
  - Dates are more complicated
    Cut up the year into irregular intervals
    But the system doesn’t change ... any more
  - Time zones are really complicated and change all the time

Time zones
    map
  - Defined in terms of Universal Coordinated Time, UTC for short
    Very similar to historical Greenwich Mean Time
  - Today, Calgary is UTC-0600
  - At 2:00 a.m. on November 5, switches to UTC-0700
  - Defined by local laws, can change with little or no notice
    Local rules changed in 2007
  - Egypt 2014: Daylight savings transitions in May, June, July, and September

timestamps/datetimes are meaningless unless you know the time zone,
    12 o’clock ... where?
and dates can be ambiguous too unless you know the time zone
    not much overlap between the workday for Australia and Vancouver

# Theory: Dates and times for computers

  - Pick any arbitrary point in time
  - Skip time zones: computers don’t care when the sunrise is
  - Turns everything into simple arithmetic
  - Number of seconds since the beginning of January 1, 1970, UTC.

# Common computations

  - What time is it now?
    humans are fine with rough answers: around 6:30
    most computer applications need precise results
    e.g., analyzing latency inside a web server
  - What time is it in ___ ?
  - What time is it here when it’s ___ in ___ ?
    for these two, need a database of time zones
  - What does the date 9/10/11 mean?
    input formatting
    human formats can be very ambiguous
    computer formats tend to be more rigidly defined
  - How long ago did ___ happen?
    output formatting
  - Remind me when ___ is about to happen
    perhaps “in one month”

# Python’s standard library

Doesn’t know about time zones!

time.strftime("%s", time.gmtime())
'1506224887'
>>> datetime.datetime.utcnow().strftime("%s")
'1506221386'
>>> datetime.datetime.now().strftime("%s")
'1506199791'
second is actually the UTC time
Python’s `0 = 'banana'` moment

mysql> SELECT 0 = 'banana';
+--------------+
| 0 = 'banana' |
+--------------+
|            1 |
+--------------+
1 row in set, 1 warning (0.00 sec)

Don’t use it.

MySQL banana

# Pendulum
  “Python datetimes made easy”

At this point, can go through sample operations one by one

automatically uses reliable time zone database

formatting: ISO8601 is actually a family of formats

# Random advice

  - ISO8601 format has some pretty nice properties: unambiguous, sortable
  - If the clock is likely to be in sync, store past events as UTC offsets
  - Store most future human-entered events as local time + timezone name,
    because the timezone definition might change

# What we haven’t covered:
  - Pendulum may or may not cover these; you’re on your own here
  - Other calendars
    macos-calendar-list.png
  - Localization
    In theory, setting your computer’s locale 1/2 or 2/1
    In practice, you should override it to YYYY-MM-DD
    But Friday, January 15, 2016
  - Leap seconds
    GPS satellites launched in 80s, require up-to-date leap second list

# Conclusion

There are many advanced date, time, and time zone topics

Pendulum handles common date, time, and time zone computations
in an easier and safer way
than Python’s built-in libraries.