Editing 2867: DateTime

{{comic
| number    = 2867
| date      = December 13, 2023
| title     = DateTime
| image     = datetime_2x.png
| imagesize = 679x478px
| noexpand  = true
| titletext = It's not just time zones and leap seconds. SI seconds on Earth are slower because of relativity, so there are time standards for space stuff (TCB, TGC) that use faster SI seconds than UTC/Unix time. T2 - T1 = [God doesn't know and the Devil isn't telling.]
}}

==Explanation==

[[Ponytail]] asks [[Cueball]] how to calculate the time elapsed between two instants. A Cueball not intimately familiar with the complexities of the way humans measure time naively assumes that this is given by the difference of the timestamps. A Cueball who is familiar panics and states that it is impossible to know, and further that it is forbidden to even ask the question.
	
Randall's use of the term "DateTime systems" covers [https://metacpan.org/pod/DateTime any] [https://docs.python.org/3/library/datetime.html number] [https://learn.microsoft.com/en-us/dotnet/api/system.datetime?view=net-8.0 of] [https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date situations], and the complexities are hardly confined to any particular programming language, or indeed computers as a whole. Some of these complexities include time zones (and changes to time zones), the international date line, daylight saving time and differing observation (or non-observation) thereof in different areas (and changes to the observation of daylight saving time over time), leap days and leap seconds, etc.  Another complexity is found in relativistic effects, in which the flow of time varies depending on how deep in a gravity well one is; {{w|Barycentric Coordinate Time}} and {{w|Geocentric Coordinate Time}} (in French, TCB and TCG respectively - the reference to TGC in the title text appears to be a typo) are time systems used for space missions and orbit calculations that handle this and shortly after this comic was published it was announced that [https://www.space.com/white-house-nasa-time-zone-moon Coordinated Lunar Time (LTC)] would be established. A partial list of such minutiae may be found at [https://gist.github.com/timvisee/fcda9bbdff88d45cc9061606b4b923ca Falsehoods Programmers Believe About Time]. The title text of the comic also references some of them. Depending on when T1 and T2 are, changes in calendar system may also be a relevant consideration—perhaps most famously, the change from the Julian calendar and the Gregorian calendar, two calendar systems which are nearly identical but nearly two weeks apart, and which different countries changed at different times.

The statement that "it is impossible to know" is because Ponytail did not provide enough information in the question: She needed to specify the location and time zone of both observations, and possibly the exact values of T1 and T2 to the nanosecond.  Considering the restrictions imposed by relativity on two observers agreeing on the timing of events, it may be literally impossible to determine a value of T2 - T1 that is absolutely "correct" to arbitrary levels of precision.

The addition of "and a sin to ask" is hyperbolic. It implies that asking such questions is akin to attempting to acquire forbidden knowledge of the nature of God or the Universe, for example, through practices such as {{w|Numerology}}, which some may consider heretical. it's an expression of the fact that determining the answer accurately can be complicated, and programming systems that attempt to do this can be frustrating.

"T2 - T1 = [God doesn't know and the Devil isn't telling.]": This is a colloquial expression that riffs on the more common "God only knows", to suggest that the thing in question is even more unknowable than the usual type of unknowable thing, to the point where it may be an evil invention of the Devil designed to cause complexity and frustration for the people having to deal with it.

==Transcript==
:[Ponytail is talking to Cueball.]
:Ponytail: Event #1 happened at time T<sub>1</sub>.
:Cueball: Okay.

:Ponytail: Then event #2 happened at time T<sub>2</sub>.
:Cueball: Mhmm.

:Ponytail: How would you calculate how much time elapsed between T<sub>1</sub> and T<sub>2</sub>?

:[The comic splits into two paths, each with a caption at the top.]

:[Path 1, upper right panel]
:Caption: Normal person:
:Cueball: T<sub>2</sub> minus T<sub>1</sub>.

:[Path 2, lower right panel]
:Caption: Anyone who's worked on datetime systems:
:[Cueball has his arms raised.]
:Cueball: '''''It is impossible to know and a sin to ask!'''''

{{comic discussion}}

[[Category:Comics featuring Ponytail]]
[[Category:Comics featuring Cueball]]
[[Category:Time]]