Date and Time MomentJS Edition

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DATE ANDTIME
Odds, Ends, and Oddities

About Me
www.maggiepint.com
@maggiepint
maggiepint@gmail.com
https://www.tempworks.com

https://xkcd.com/1597/

Like Git:
• Date and time is complicated
• At first, we avoid learning date and time
But also, like Git:
• Date and time m...

5 O'clock Somewhere

PERSPECTIVE
We all see dates and times from different angles

The GlobalTimeline
1 2 3 4 5 6 7 8 9
Point in absolute time

Coordinated UniversalTime (UTC)
• A perspective of the global timeline
• Allows us to keep track of absolute time
• Primar...

LocalTime
• A local time is a perspective of time
• It does not represent a point on the global timeline
• It is usually n...

UTC Time:
2016-04-09T14:17:47Z
What we know
• The point in absolute time
• Whether this time is before or after
another po...

LocalTime:
Saturday,April 9, 2016 9:11 AM
We Know
• It is Saturday
• It is morning
We Don’t Know
• What point this is on t...

TIME ZONES
Uniting Perspectives

Time Zone Basics
• A time zone is a region that observes a uniform standard time
• A time zone is defined by a set of offs...

Time Zone: America/Chicago

Politics
• Time zones are inherently political
• Governments change time zones regularly
• Governments change time zones s...

IANATime Zone Libraries
• JavaScript - MomentTimeZone
• http://momentjs.com/timezone/
• .NET – NodaTime
• http://nodatime....

Time Zones are not Offsets!
"2016-04-09T19:39:00-05:00“
This date could be in:
• America/Chicago
• America/Bahia_Banderas
...

ASSUMPTIONS
Things we think we know

Assumption
“If you just store everything in UTC, all your
problems will be solved.”

ATableWith Everything in UTC

Show me all the messages for the
business day of April 4th.
I have users in London, and across the United States.
In Londo...

Perspectives
• When storing a date, consider the following perspectives
• Absolute time
• Time local to the date’s viewer
...

2016-04-09T20:18:48-05:00
ISO 8601 Format with Offset
Local Date LocalTime Offset

Why use ISO 8601 format?
• With offset, reflects both local and absolute perspective
• Has unambiguous ordering
• Compare ...

Assumption
“If I just store everything in ISO 8601 with an
offset, everything will be fine.”

Future Dates
• Time zones change over time
• In the future, the offset of a scheduled time could change
• Store future dat...

Assumption
“There are 24 hours in a day, and 365 days in a
year.”

moment('2016-03-12 12:00').add(1, 'day').format('LLL')
"March 13, 2016 12:00 PM"
moment('2016-03-12 12:00').add(24,'hour')...

moment('2016-02-28').add(365, 'days').format('LLL')
"February 27, 2017 12:00 AM"
moment('2016-02-28').add(1, 'year').forma...

As Seen on Stack Overflow
var startHours = 8;
var startMinutes = 30;
var ed = new Date();
var endHours = ed.getHours();
va...

Assumption
“Time and date math work in the same way.”

moment('2016-01-01').add(1.5, 'hours').format('LLL')
"January 1, 2016 1:30AM“
moment('2016-01-01').add(1.5, 'days').format...

Time Math vs Date Math
Time math:
• Refers to operations involving hours, minutes, seconds, milliseconds
• Works by increm...

Assumption
“All dates and times exist once in all time
zones”

moment('2016-10-16').format('LLL')
October 16, 2016 1:00 AM

Spring Forward in JavaScript

Fall Back in JavaScript

Samoa
• Samoa is very close to the international date line
• Samoa found itself trading more frequently with Australia tha...

Assumption
“The Date object in JavaScript works.”

var a = new Date('2016-01-01');
a.toString();
"Thu Dec 31 2015 18:00:00 GMT-0600 (Central StandardTime)"
var a = new Date(...

Known JavaScript Date Issues
• DSTTransitions can go both directions
• Months index from zero
• No support for time zones ...

MOMENT.JS
Quality Date andTime in Javascript

Parsing
• Consistently parses date values according to your expectations
• More than 100 languages supported
• Thousands o...

moment('2016-12-21T13:25:22').format()
"2016-12-21T13:25:22-06:00"
moment('30/04/2016', 'DD/MM/YYYY').format()
"2016-04-30...

Formatting
• Highly configurable format options
• Locale based formatting functions

moment().format('MM/DD/YYYY')
"04/26/2016"
moment().format('MMMM D, YYYY hh:mm a')
"April 26, 2016 09:26 pm"

moment().format('LLL')
"April 26, 2016 9:28 PM"
moment().format('L')
"04/26/2016"
moment().locale('en-gb').format('L')
"26...

Sane Math
• Add and subtract any unit
• StartOf/EndOf
• Difference

var a = new Date();
a.setDate(a.getDate() - 5);
moment().subtract(5, 'days');

var d = new Date();
var diff = d.getDate() - d.getDay() + (day == 0 ? -6:1);
d.setDate(diff);
d.setHours(0,0,0,0);
moment(...

var d1 = new Date();
var d2 = new Date(01/01/2016);
var months;
months = (d2.getFullYear() - d1.getFullYear()) * 12;
month...

RelativeTime
moment().subtract(3, 'months').fromNow()
"3 months ago"
moment().subtract(2, 'seconds').fromNow()
"a few seco...

Time Zones (With MomentTimeZone)
moment.tz('2016-04-25T02:30:00Z', 'America/Los_Angeles').format()
"2016-04-24T19:30:00-07...

IN SUMMARY
We all know what happens when you assume

CONSIDER ALL PERSPECTIVES

DISTINGUISH BETWEEN LOCAL AND
ABSOLUTETIME

REMEMBER,TIME ZONES CHANGE
RAPIDLY

STORE FUTURE DATES IN LOCALTIME

DISTINGUISH BETWEEN DATE MATH
ANDTIME MATH

DON’TTRUST JAVASCRIPT DATE

USE A QUALITY LIBRARY

MAKE NO ASSUMPTIONS

Be like this guy:

Additional Resources
• Date andTime Fundamentals – Matt Johnson, Pluralsight
• https://www.pluralsight.com/courses/date-ti...

About Me
www.maggiepint.com
@maggiepint
maggiepint@gmail.com
https://github.com/maggiepint
https://www.tempworks.com

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