Which Time Is It?

Can you add two time values together? Yes. No. Not so fast!

Reset your clocks and join me on a graphical tour of time itself. You'll discover how "time" is more than a single thing, build intuition around what different operations mean, and get a sense of when some operations are nonsensical. You'll leave with a better mental model for thinking about time and avoiding subtle time-related bugs in your own code.

Originally given at RubyConf 2023

Transcript

1. Which Time Is It?
2. "Time" has many meanings in English
3. Choosing the right time: Better design and avoids bugs
4. What is the difference between 1.day and 1.day.ago?
5. Two Kinds of Time Moment November 14th, 2023 at 10:30AM PST Duration 45 minutes
6. Visualizing the difference
7. Timeline
8. Moment Points on the timeline
9. Duration Distance on the timeline
10. Not 1d vectors!
11. CLASSES PROVIDED BY Ruby
12. MOMENT DateTime
13. MOMENT DateTime Deprecated
14. MOMENT (NANOSECOND RESOLUTION) Time Nov 14, 2023 10:30:05.123456789 AM PST
15. Time Nanoseconds apart
16. MOMENT (DAY RESOLUTION) Date Nov 14, 2023
17. Date Days apart
18. Resolution impacts enumeration start = Date.new(2023, 11, 12) finish = Date.new(2023, 11, 15) (start..finish).map(&:to_s) # 2023-11-12 # 2023-11-13 # 2023-11-14 # 2023-11015
20. Notice: It's all the same kind of time!
21. Date/Time for moments and numerics for durations Core Ruby represents time with
22. Math! Not all operations make sense
23. Time docs Interesting method signatures
24. Time#+ time + numeric -> time
25. time + numeric -> time Notice: We can't add two time instances!
26. What question is this asking?
27. What point is 45 mins after point A? ADDITION IS ASKING:
28. Time#+ What point is 45 mins after point A?
29. Time#- time - numeric -> time time - other_time -> float
30. Notice: there are 2 signatures!
31. What questions is this asking?
32. What point is 45 mins before point A? SUBTRACTION IS ASKING:
33. time - numeric -> time What point is 45 mins before point A?
34. time - time -> float Notice: we can subtract two points!
35. What questions is this asking?
36. How big is the duration between points A and B? SUBTRACTION IS ASKING:
37. time - time -> float What is the distance between points A and B?
38. 3 questions we can answer about the same data
39. Same equation, rearanged point1 + duration = point2 (Time#+) point2 - duration = point1 (Time#- v1) point2 - point1 = duration (Time#- v2)
40. What it you need a time without an associated day?
41. Other Kinds of Time Moment November 14th, 2023 at 11:30AM PST Duration 45 minutes Time of Day 5:30 PM
42. Time of Day Points on a circular timeline
43. How does ToD interact with other types?
44. Time of Day Our 3 questions still work!
45. Extract from a Time
46. Combine ToD with Date to get a Time
47. Third Party Solutions GEMS!
48. DURATION ActiveSupport third party gems
49. DURATION 1.day
50. POINT IN TIME 1.day.ago
51. Sugar Time.now - (1 * SECONDS_PER_DAY)
52. TIME OF DAY jackc/tod third party gems
53. How to Implement Time?
54. Warning
55. Avoid human-readable integers like 1030
56. Invalid times like 3176
57. Breaks Math (1025 - 0925 = 100)
58. Avoid multi-part values like {hour: 10, minute: 30}
59. Epoch + Counter
60. Epoch: when is 0?
61. Counter: how many <resolution> since 0?
62. Common Implementations UNIX Time milliseconds since Jan 1, 1970 Postgres Time of Day microseconds since midnight Ruby Date days since Jan 1, 4713 BCE
63. Just a big counter class TimeOfDay def initialize(microseconds_since_midnight) @counter = (microseconconds_since_midnight % MS_PER_DAY) end end
64. Custom constructor class TimeOfDay def self.from_parts(hours:, minutes:, seconds:) total = (hours * MICROSECONDS_PER_HOUR) + (minutes * MICROSECONDS_PER_MINUTE) + (seconds * MICROSECONDS_PER_SECOND) new(total) end end
65. Operators class TimeOfDay def +(duration_microseconds) self.class.new(@counter + duration_microseconds) end end
66. Case Study
68. When working with time Think About What kind of time you need moment, duration, tod, etc What operations make sense (and which ones don't!) both within and across types What resolution you need e.g. Time vs Date class
69. Joël Quenneville Joël Quenneville Principal Developer thoughtbot @joelquen bikeshed.fm co-host