DATE AND TIMEOdds, Ends, and Oddities

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Like Git:• Date and time is complicated• At first, we avoid learning date and time

But also, like Git:• Date and time makes sense• Date and time can be fun

5 O'clock Somewhere

PERSPECTIVEWe all see dates and times from different angles

The Global Timeline

1 2 3 4 5 6 7 8 9

Point in absolute time

Coordinated Universal Time (UTC)• A perspective of the global timeline• Allows us to keep track of absolute time• Primary standard by which the world regulates clocks• Defined precisely by scientific community• Includes leap seconds• Has no relationship to geography• Basically the same as GMT, but GMT is not defined precisely by the

scientific community

Local Time• A local time is a perspective of time• It does not represent a point on the global timeline• It is usually not a contiguous timeline (DST)

UTC Time:2016-04-09T14:17:47ZWhat we know• The point in absolute time• Whether this time is before or

after another point in time

What we don’t know• Whether it is morning or night• What day of the week it is

Local Time:Saturday, April 9, 2016 9:11 AM

We Know• It is Saturday• It is morning

We Don’t Know• What point this is on the

global timeline• Whether it is before or after a

time from a different locality• What the time will be if we

add one hour to this time

TIME ZONESUniting 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 offsets from UTC• Offsets are typically in one hour intervals, but not always

• Nepal Standard Time is UTC +5:45• Time zones frequently have multiple offsets• There are two main resources for time zones

• Internet Assigned Numbers Authority (IANA)• Windows Time Zones

• IANA time zones are more widely used, and more accurate

Time Zone: America/Chicago

Politics• Time zones are inherently political• Governments change time zones regularly• Governments change time zones suddenly• Russia this year is an example of politicians causing time chaos

• http://bit.ly/1SB9TvW• Morocco reverts to standard time during Ramadan• Assume nothing

IANA Time Zone Libraries• JavaScript - Moment TimeZone

• http://momentjs.com/timezone/• .NET – NodaTime

• http://nodatime.org/• Java 8 + - java.time (native)

• Java 7 - JodaTime• See Stack Overflow post for more exhaustive list

• http://bit.ly/1RUYuuM

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

ASSUMPTIONSThings we think we know

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

A Table With 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 London April 4th is between 2016-04-03 23:00:00Z and 2016-04-04 22:59:59ZIn Minneapolis April 4th is between 2016-04-04 05:00:00Z and 2016-04-05 04:59:59Z

Perspectives• When storing a date, consider the following perspectives

• Absolute time• Time local to the date’s viewer• Time local to the date’s originator

2016-04-09T20:18:48-05:00

ISO 8601 Format with Offset

Local Date Local Time Offset

Why use ISO 8601 format?• With offset, reflects both local and absolute

perspective• Has unambiguous ordering• Compare this to 4/10/2016• In the US this is April 10th

• In the UK this is October 4th • Helps avoid having to compute local perspective from

absolute during querying• Supported by nearly all modern databases• Databases will automatically order in absolute time

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 dates in local time with a time zone

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').format('LLL')

"March 13, 2016 1:00 PM"

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

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

As Seen on Stack Overflowvar startHours = 8;var startMinutes = 30;

var ed = new Date();var endHours = ed.getHours();var endMinutes = ed.getMinutes();

var elapsedMinutes = (endHours * 60 + endMinutes) - (startHours * 60 + startMinutes);

console.log(elapsedMinutes);

Assumption“Time and date math work in the

same way.”

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

moment('2016-01-01').add(1.5, 'days').format('LLL')"January 3, 2016 12:00 AM"

Time Math vs Date MathTime math:• Refers to operations involving hours, minutes, seconds, milliseconds• Works by incrementing or decrementing the position on the global timeline by the

number of units in question• Can use fractional units

Date Math:• Refers to all operations larger than hours – days, months, years, quarters, etc.• Works by moving places on the calendar • Cannot be converted to time math• Cannot use fractional units

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

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 Standard Time)"

var a = new Date('1/1/2016');a.toString();

"Fri Jan 01 2016 00:00:00 GMT-0600 (Central Standard Time)"

Known JavaScript Date Issues• DST Transitions can go both directions• Months index from zero• No support for time zones other than user’s local time zone and

UTC• Older browsers know only current DST rules• Parsing is implementation specific and basically completely broken

• The spec is a disaster

http://codeofmatt.com/2013/06/07/javascript-date-type-is-horribly-broken/

IN SUMMARYWe all know what happens when you assume

CONSIDER ALL PERSPECTIVES

DISTINGUISH BETWEEN LOCAL AND ABSOLUTE TIME

REMEMBER, TIME ZONES CHANGE RAPIDLY

STORE FUTURE DATES IN LOCAL TIME

DISTINGUISH BETWEEN DATE MATH AND TIME MATH

DON’T TRUST JAVASCRIPT DATE

USE A QUALITY LIBRARY

MAKE NO ASSUMPTIONS

Be like this guy:

Additional Resources• Date and Time Fundamentals – Matt Johnson, Pluralsight

• https://www.pluralsight.com/courses/date-time-fundamentals• Matt Johnson’s Blog

• http://codeofmatt.com/• Lau Taarnskov’s Blog

• http://www.creativedeletion.com/• NodaTime’s Documentation

• http://nodatime.org/• Time Programming Fundamentals – Greg Miller CPPCon 2015

• http://bit.ly/1SgO3E0• Moment.js

• http://momentjs.com/

About Mewww.maggiepint.com

@maggiepint

maggiepint@gmail.com

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https://www.tempworks.com