Some developers write date/time-sensitive code without worrying about it — but also without thinking about it. Some developers write date/time-sensitive code and worry about it a lot because it's hard. In this talk Jon will provide some guidance to help you write date/time-sensitive code with a reasonable degree of confidence, and test it. We'll discuss fundamental concepts, and how they map onto both .NET's out-of-the-box types, and the types in Noda Time (an open source project which Jon maintains).

2. Agenda
• Intro
• Concepts
• How to use the concepts
• Gotchas, myths and anti-patterns
• Recommendations

3. Non-agenda
• Relativity
• Leap seconds

5. Concepts
• Instant
• Duration (TimeSpan)
• Interval
• Calendar system (CalendarSystem, Calendar)
• Date (LocalDate, DateOnly)
• Time of day (LocalTime, TimeOnly)
• Date and time (LocalDateTime, DateTime)
• Time zones (DateTimeZone, TimeZoneInfo)
• Date and time in a zone (ZonedDateTime)
• Date and time with offset (OffsetDateTime, DateTimeOffset)
• Period

6. Hold on! Why not just use DateTime
• DateTime.Kind makes it hard to reason about code
• Imagine only having System.Double for numbers
• DateTimeOffset can be useful, but it’s not a silver
bullet
• DateOnly and TimeOnly are welcome recent
additions

7. Instant

8. Duration (TimeSpan)

9. Interval

10. Calendar system (CalendarSystem, Calendar)

11. Date (LocalDate, DateOnly)

12. Time of day (LocalTime, TimeOnly)

13. Date and time (LocalDateTime, DateTime)

14. Time zones (DateTimeZone, TimeZoneInfo)

15. Date and time in a zone (ZonedDateTime)

16. Date and time with offset
(OffsetDateTime, DateTimeOffset)

17. Period

18. How to use the concepts
• Arithmetic: calendrical vs timeline
• Choosing what to store
• Conversions between local and zoned
• Being aware of calendars and cultures
• Textual representations
• Testability: zone and clock dependencies

19. Top tip: inject a clock
(copy this + add a fake, or use NodaTime)
public interface IClock
{
DateTimeOffset GetCurrentInstant();
}
public static class Clock : IClock
{
public static Clock Instance { get; } = new Clock();
private Clock() {}
public DateTimeOffset GetCurrentInstant() =>
DateTimeOffset.UtcNow;
}

20. Tricky bits
•Incomplete information
(“which 1:35am did you mean?”)
•Limitations of DateTime
•Time zone rule changes

21. Gotchas, myths, anti-patterns
“Only store UTC”
“I can assume the server is in time zone X…”
“It’s fine to use time zone abbreviations…”
“I only do a bit of date/time work, I’ll roll my own”
“I know that the time I’m converting will never be
ambiguous”

22. Recommendations
• Put some time in. It’s all feasible.
• Find the right tool for the job
• Be deliberate: think (and document) before
you code
• Lots and lots of tests
• Subscribe to the IANA Time Zone
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