Dates and Times by Example (AltConf 2018)

Have you ever had a bug that dealt with dates and times? Dealing with time zones, Daylight Saving Time, and the like can make the hair on the back of even the most experienced developer's neck stand up. We all know how to tell time and how calendars work, and yet these bugs are terrifying. Why? Using Swift as a lens to walk through common problems, we’ll explore writing date and time code that’s correct and easy to understand. You’ll leave with an approach to solving calendrical calculations without fear, new tools for handling these problems in your code, and a huge appreciation for the people behind the libraries that make this possible.

Transcript

1. Dates and Times by Example
   Jeff Kelley @SlaunchaMan
   

   View Slide

2. About Me
   • iOS Developer for 10 years
   • Recently: web frontend/middleware
   using TypeScript
   • Author of Learn Cocoa Touch for iOS
   and Developing for Apple Watch
   Jeff Kelley @SlaunchaMan
   

   View Slide

3. About Me
   • iOS Developer for 10 years
   • Recently: web frontend/middleware
   using TypeScript
   • Author of Learn Cocoa Touch for iOS
   and Developing for Apple Watch
   Jeff Kelley @SlaunchaMan
   

   View Slide

4. About Me
   • iOS Developer for 10 years
   • Recently: web frontend/middleware
   using TypeScript
   • Author of Learn Cocoa Touch for iOS
   and Developing for Apple Watch
   Jeff Kelley @SlaunchaMan
   

   View Slide

5. I still struggle with date and time issues.
   Jeff Kelley @SlaunchaMan
   

   View Slide

6. Previous Talks
   • Dates and Times in iOS, AltConf 2014
   • Advanced Dates and Times in Swift, 360|iDev 2017
   Jeff Kelley @SlaunchaMan
   

   View Slide

7. Quick Recap
   • A Date is a moment in time.
   • Stop thinking about Date in terms of time zones
   • When you po a Date, you get something like this:
   2018-06-05 20:26:22 +0000
   Jeff Kelley @SlaunchaMan
   

   View Slide

8. Quick Recap
   • A Date is a moment in time.
   • Stop thinking about Date in terms of time zones
   • When you po a Date, you get something like this:
   2018-06-05 20:26:22 +0000
   - timeIntervalSinceReferenceDate :
   549923182.35240304
   Jeff Kelley @SlaunchaMan
   

   View Slide

9. Formatters
   • Use them!
   • We have gotten new formatters since NSDateFormatter,
   check them out to see if they meet your needs.
   Jeff Kelley @SlaunchaMan
   

   View Slide

10. Demo: Formatting Intervals
    Jeff Kelley @SlaunchaMan
    

    View Slide

11. Problems With Solutions
    Jeff Kelley @SlaunchaMan
    

    View Slide

12. Problems With Solutions
    This talk will have two main parts: problems with solutions,
    and problems without solutions. The previous example was
    the first case.
    Jeff Kelley @SlaunchaMan
    

    View Slide

13. Time Zones and Locations
    Jeff Kelley @SlaunchaMan
    

    View Slide

14. View Slide

15. Demo: Time Zones and Locations
    Jeff Kelley @SlaunchaMan
    

    View Slide

16. Demo: Time Zone-Agnostic Notifications
    Jeff Kelley @SlaunchaMan
    

    View Slide

17. Problems Without Solutions
    Jeff Kelley @SlaunchaMan
    

    View Slide

18. Problems Without Solutions
    What if there's no “right answer?”
    Jeff Kelley @SlaunchaMan
    

    View Slide

19. Problems Without Solutions
    It’s 11:55 PM. Your user says, “In fifteen minutes, remind me to
    go to bed.” But there’s a leap second at 11:59:60. Does the
    timer go off at 12:10:00 AM or 12:09:59 AM?
    Jeff Kelley @SlaunchaMan
    

    View Slide

20. Problems Without Solutions
    Year-Over-Year Graphs
    Jeff Kelley @SlaunchaMan
    

    View Slide

21. Problems Without Solutions
    Year-Over-Year Graphs
    Jeff Kelley @SlaunchaMan
    

    View Slide

22. Problems Without Solutions
    Year-Over-Year Graphs
    Jeff Kelley @SlaunchaMan
    

    View Slide

23. Problems Without Solutions
    Month-over-Month Graphs
    Jeff Kelley @SlaunchaMan
    

    View Slide

24. Problems Without Solutions
    Week-over-Week Graphs
    Jeff Kelley @SlaunchaMan
    

    View Slide

25. How We Talk About Dates
    “We entered the timestamps in the location’s time zone.”
    TIMESTAMP 2018-06-07 14:45
    Jeff Kelley @SlaunchaMan
    

    View Slide

26. How We Talk About Dates
    TIMESTAMP 2018-06-07 14:45 AT TIMEZONE 'PDT'
    TIMESTAMP 2018-06-07 14:45 AT TIMEZONE 'America/
    Los_Angeles'
    Jeff Kelley @SlaunchaMan
    

    View Slide

27. Demo: Date Ranges with DateInterval
    Jeff Kelley @SlaunchaMan
    

    View Slide

28. More Resources
    • Date and Time Programming Guide, Apple
    • YourCalendricalFallacyIs.com, Dave DeLong
    • Chronology, Dave DeLong
    Jeff Kelley @SlaunchaMan
    

    View Slide

29. Thank You!
    Jeff Kelley, jeff@detroitlabs.com
    @SlaunchaMan on Twitter
    

    View Slide