Blame it on Caesar!

Lennart Regebro

PyCon US 2013, Santa Clara, Grand Ballroom AB

Grand Ballroom AB, eh?

Why not in the "Great America" Ballroom?

I heard America was great.

It was smaller than I expected.

Didn't look that special either.

Calendaring

Measuring time in celestial cycles

Roman calendars

You name 'em, they had 'em

Calendar of Romulus

10 lunar months
2-3 uncounted lunar months!
Works for farming
Not so great for business

Lunar calendars

The year is 12 lunar months long
The year is out of sync with the seasons
Other examples: The Islamic calendar

Lunisolar calendars

The year is 12 or 13 lunar months long
Leap months keep the year in sync
Other examples: The Hebrew, Buddhist, Burmese, Babylonian, etc, etc...

Villain #1

Gaius Julius Caesar

Solar calendars

The year follows the solstices/seasons
The moon is ignored completely
Examples: The French republican calendar, the Julian Calendar, the Ancient Egyptian calendar...

My calendar brings all the boys to the yard!

And they're like, it's better than yours.

Damn right, it's better than yours!

I could teach you, but I'd have to charge.

Spot the patterns

31 28 31 30 31 30 31 31 30 31 30 31

31 29 31 30 31 30 31 31 30 31 30 31

Leap Day!

February 24th!

Don't ask why

It's the 29th now

Except in Hungary

Weeks

Egyptian calendar math

def day_of_year(month, day):
    return (month - 1) * 30 + day_of_month

def day_of_week(day):
    return ((day - 1) % 10) + 1

def weekno(month, day):
    return ((day_of_year(month, day) - 1) // 10) + 1

Julian calendar math

MONTH_LENGHTS_STD = [0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334]
MONTH_LENGHTS_LYR = [0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335]
MONTH_DAY_OF_WEEK = [0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4]

def day_of_year(year, month, day):
    if not year % 4 and year % 100:
        d = MONTH_LENGHTS_LYR[month-1]
    else:
        d = MONTH_LENGHTS_STD[month-1]
    return d + day

def day_of_week(year, month, day):
    if month < 3:
        year -= 1
    dow = (year + year//4 - year//100 + year//400 +
           MONTH_DAY_OF_WEEK[month-1] + day) % 7
    if dow == 0:
        dow = 7
    return dow

def _weekno(year, month, day):
    return (day_of_year(year, month, day) -
            day_of_week(year, month, day) + 10) // 7

def weekno(year, month, day):
    week = _weekno(year, month, day)
    if week == 0:
        week = _weekno(year - 1, 12, 31)
    if week == 53:
        if _weekno(year + 1, 1, 1) == 1:
            week = 1
    return week

Calendaring and you

There is calendaring

bug born every day!

Always wear a library!

Python

`datetime`

Javascript

Javascript has zero-indexed months

January is 0

December is 11

Villain #2

James Gosling; created Java

You put the first day

into the week end?

Python and weekdays

`weekday()` uses `0` for Monday

ISO and weekdays

`isoweekday()` uses `1` for Monday

Always use 1 for Monday

And 7 for Sunday

Or possibly 0

But never ever 6

Time zones

A brief history of something that doesn't exist

Villain #3

William Willet; invented DST

BST: 5 different zones
CST: 4 different zones
IST: 4 different zones

America/Chicago
Australia/Canberra
Asia/Shanghai
Asia/Taipei

Midnight DST change

You are scaring the computer

>>>> from datetime import date
>>> date(2011,  2, 29)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: day is out of range for month

js> new Date(2011, 1, 29);
Tue Mar 01 2011 00:00:00 GMT+0100 (CET)

js> y = 2011; m = 1; d = 29;
js> dt = new Date(y, m, d);
js> if (dt.getFullYear() !== y ||
        dt.getMonth() !== m ||
        dt.getDay() !== d) {
  > (Insert error handling here)
  > }

$ TZ='Europe/Rome' js
Rhino 1.7 release 3 2012 02 16
js> new Date(1972, 4, 28);
Sat May 27 1972 23:00:00 GMT+0100 (CET)

js> new Date(y, m, d, 12);
Sun May 28 1972 12:00:00 GMT+0200 (CEST)

Python & Timezones

Python 2.3 - 3.1

Abstract class tzinfo only

Python 3.2 - 3.3

timezone class for fixed offsets

Python 3.4?

PEP-431: Full zoneinfo support

`pytz` or `dateutil`?

Let's have a shootout!

`pytz` has the latest zoneinfo database

`pytz` 15 - 0 `dateutil`

`dateutil` will use the OS database

`pytz` 15 - 15 `dateutil`

But not on Windows

`pytz` 30 - 15 `dateutil`

`dateutil` can parse dates with time zones

`pytz` 30 - 30 `dateutil`

Which half past two do you mean?

>>> rome = pytz.timezone('Europe/Rome')
>>> rome.localize(datetime(1972, 5, 28, 0, 0), is_dst=None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/dist-packages/pytz/tzinfo.py",
    line 322, in localize
    raise NonExistentTimeError(dt)
pytz.exceptions.NonExistentTimeError: 1972-05-28 00:00:00
>>>

`pytz` has better DST handling

`pytz` 40 - 30 `dateutil`

`dateutil` has better local time zone support

`pytz` 40 - 40 `dateutil`

`tzlocal`

Local time zone support for pytz

http://pypi.python.org/pypi/tzlocal

Advantage `pytz`

PEP-431 is based on `pytz`

But not the API

Parsing timezones

RFC 822 format

>>> from dateutil.parser import parse

>>> parse('2003-09-25 12:45 -0300')
datetime.datetime(2003, 9, 25, 12, 45,
        tzinfo=tzoffset(None, -10800))

POSIX format

>>> parse('2003-09-25 12:45 GMT-0300')
datetime.datetime(2003, 9, 25, 12, 45,
        tzinfo=tzoffset(None, 10800))

Wait what?

iCalendar

RFC 5545 / RFC 2445

The Python Secret Underground meeting schedule

First meeting: Saturday 28th of August 1999

Every Last Saturday of the month, except in August, when it's the 28th. And also, every 13th of every month, unless it's a Friday.

	SECONDLY	MINUTELY	HOURLY	DAILY	WEEKLY	MONTHLY	YEARLY
BYMONTH	Limit	Limit	Limit	Limit	Limit	Limit	Expand
BYWEEKNO	N/A	N/A	N/A	N/A	N/A	N/A	Expand
BYYEARDAY	Limit	Limit	Limit	N/A	N/A	N/A	Expand
BYMONTHDAY	Limit	Limit	Limit	Limit	N/A	Expand	Expand
BYDAY	Limit	Limit	Limit	Limit	Expand	Note 1	Note 2
BYHOUR	Limit	Limit	Limit	Expand	Expand	Expand	Expand
BYMINUTE	Limit	Limit	Expand	Expand	Expand	Expand	Expand
BYSECOND	Limit	Expand	Expand	Expand	Expand	Expand	Expand
BYSETPOS	Limit	Limit	Limit	Limit	Limit	Limit	Limit

dateutil.rrule

Implements all this so you don't have to!

Using a timezone in iCalendar

DTSTART;TZID=America/New_York:19980119T020000

You must define the zone

BEGIN:VTIMEZONE
TZID:America/New_York
LAST-MODIFIED:20050809T050000Z
BEGIN:DAYLIGHT
DTSTART:19670430T020000
RRULE:FREQ=YEARLY;BYMONTH=4;BYDAY=-1SU;UNTIL=19730429T070000Z
TZOFFSETFROM:-0500
TZOFFSETTO:-0400
TZNAME:EDT
END:DAYLIGHT
BEGIN:STANDARD
DTSTART:19671029T020000
RRULE:FREQ=YEARLY;BYMONTH=10;BYDAY=-1SU;UNTIL=20061029T060000Z
TZOFFSETFROM:-0400
TZOFFSETTO:-0500
TZNAME:EST
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:19740106T020000
RDATE:19750223T020000
TZOFFSETFROM:-0500
TZOFFSETTO:-0400
TZNAME:EDT
END:DAYLIGHT
BEGIN:DAYLIGHT
DTSTART:19760425T020000
RRULE:FREQ=YEARLY;BYMONTH=4;BYDAY=-1SU;UNTIL=19860427T070000Z
TZOFFSETFROM:-0500
TZOFFSETTO:-0400
TZNAME:EDT
END:DAYLIGHT
BEGIN:DAYLIGHT
DTSTART:19870405T020000
RRULE:FREQ=YEARLY;BYMONTH=4;BYDAY=1SU;UNTIL=20060402T070000Z
TZOFFSETFROM:-0500
TZOFFSETTO:-0400
TZNAME:EDT
END:DAYLIGHT
BEGIN:DAYLIGHT
DTSTART:20070311T020000
RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=2SU
TZOFFSETFROM:-0500
TZOFFSETTO:-0400
TZNAME:EDT
END:DAYLIGHT
BEGIN:STANDARD
DTSTART:20071104T020000
RRULE:FREQ=YEARLY;BYMONTH=11;BYDAY=1SU
TZOFFSETFROM:-0400
TZOFFSETTO:-0500
TZNAME:EST
END:STANDARD
END:VTIMEZONE

Good enough

DTSTART:19980119T080000Z
DTSTART:19980119T020000

The `icalendar` module

No Python 3 support (yet)
Bugs in advanced cases
Good enough

Further considerations

Events with or without time zones?
Or both?
Different time zone for start and end?
Whole-day events?

User interface

Local display formats
Event lists: Upcoming / previous
Day / week / month / year views
First day of week support

Date inputs: The text field

Fast entry
Input format always a problem

The date picker

Most common solution
Not accessible
Requires Javascript
Do not allow you to type in dates

The drop down list

Slow to use
Accessible
Doesn't allow you to type in dates

jquery.recurrenceinput

https://github.com/collective/jquery.recurrenceinput.js

http://regebro.wordpress.com/

Creaditus Generalus

Presentation software:

Hovercraft

https://github.com/regebro/hovercraft

Fonts:

Cinzel by Natanael Gama

DejaVu Mono Book

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P	Open presenter console
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Blame it on Caesar!

Lennart Regebro

Calendaring

Measuring time in celestial cycles

Roman calendars

You name 'em, they had 'em

Calendar of Romulus

Lunar calendars

Lunisolar calendars

Villain #1

Gaius Julius Caesar

Solar calendars

Spot the patterns

31 28 31 30 31 30 31 31 30 31 30 31

31 28 31 30 31 30 31 31 30 31 30 31

31 28 31 30 31 30 31 31 30 31 30 31

31 29 31 30 31 30 31 31 30 31 30 31

Leap Day!

February 24th!

Don't ask why

It's the 29th now

Except in Hungary

Weeks

Egyptian calendar math

Julian calendar math

Calendaring and you

There is calendaring

bug born every day!

Always wear a library!

Python

datetime

Javascript

Javascript has zero-indexed months

Villain #2

James Gosling; created Java

You put the first day

into the week end?

Python and weekdays

weekday() uses 0 for Monday

ISO and weekdays

isoweekday() uses 1 for Monday

Always use 1 for Monday

And 7 for Sunday

Or possibly 0

But never ever 6

Time zones

A brief history of something that doesn't exist

Villain #3

William Willet; invented DST

Midnight DST change

You are scaring the computer

Python & Timezones

Python 2.3 - 3.1

Python 3.2 - 3.3

Python 3.4?

pytz or dateutil?

pytz has the latest zoneinfo database

pytz 15 - 0 dateutil

dateutil will use the OS database

pytz 15 - 15 dateutil

But not on Windows

pytz 30 - 15 dateutil

dateutil can parse dates with time zones

pytz 30 - 30 dateutil

Which half past two do you mean?

pytz has better DST handling

pytz 40 - 30 dateutil

dateutil has better local time zone support

pytz 40 - 40 dateutil

tzlocal

Advantage pytz

PEP-431 is based on pytz

Parsing timezones

RFC 822 format

POSIX format

iCalendar

RFC 5545 / RFC 2445

The Python Secret Underground meeting schedule

dateutil.rrule

Using a timezone in iCalendar

`datetime`

`weekday()` uses `0` for Monday

`isoweekday()` uses `1` for Monday

`pytz` or `dateutil`?

`pytz` has the latest zoneinfo database

`pytz` 15 - 0 `dateutil`

`dateutil` will use the OS database

`pytz` 15 - 15 `dateutil`

`pytz` 30 - 15 `dateutil`

`dateutil` can parse dates with time zones

`pytz` 30 - 30 `dateutil`

`pytz` has better DST handling

`pytz` 40 - 30 `dateutil`

`dateutil` has better local time zone support

`pytz` 40 - 40 `dateutil`

`tzlocal`

Advantage `pytz`

PEP-431 is based on `pytz`

The `icalendar` module