Implementation of operation (TimeOnly - TimeOnly)

[dmitry-lipetsk]

Hello,

runtime/src/libraries/System.Private.CoreLib/src/System/TimeOnly.cs

Lines 249 to 255 in d2cef2f

	/// <summary>
	/// Gives the elapsed time between two points on a circular clock, which will always be a positive value.
	/// </summary>
	/// <param name="t1">The first TimeOnly instance.</param>
	/// <param name="t2">The second TimeOnly instance..</param>
	/// <returns>The elapsed time between t1 and t2.</returns>
	public static TimeSpan operator -(TimeOnly t1, TimeOnly t2) => new TimeSpan((t1._ticks - t2._ticks + TimeSpan.TicksPerDay) % TimeSpan.TicksPerDay);

Could anybody explain (or point the exists discussion) the reason of this implementation?

1. If you want return a TimeSpan (interval between time), then result may be negative. Because if (A < B), then (A - B)<0

2. If you want return a TIME (as you are say in comment "Gives the elapsed time between two points"), then result must be TimeOnly.

Current implementation returns time in data type for interval.

It is correct?

---

Reason of this question.

In provider for EFCore need map this operation to SQL.

DBMS (PG, Firebird) for expression

time '01:00:00' - time '03:00:00'

returns negative value: -02:00:00.

Subtract operator of TimeOnly returns positive value "22:00:00".

---

Thanks.

[Clockwork-Muse]

If you want return a TimeSpan (interval between time), then result may be negative. Because if (A < B), then (A - B)<0

In the ISO world (and in reality), there is no such thing as a negative duration. (Although I personally agree with you here, that it does make some operations easier). This was debated at design time, and changing it now would be a massive breaking change.

If you want return a TIME (as you are say in comment "Gives the elapsed time between two points"), then result must be TimeOnly.

"Time" has multiple uses in English - the phrase "time me", for instance, is asking for a duration, not the wall-clock time. Hence why it's "elapsed time" in the method documentation.

Reason of this question.

If the values are subtracted on the database side, the value returned will be a negative timestamp. If you're emulating this in the EFCore mapping, you need to add an extra check.

[bartonjs]

If you want return a TIME (as you are say in comment "Gives the elapsed time between two points"),

"elapsed time" is the same as "a span of time", so TimeSpan is correct for that one.

TimeOnly is half of DateTime (the time half). It represents values that can be seen on a 24h clock. 23:59:59 + one second is 00:00:00. It's for "this meeting is at 3, every Friday".

[svick]

When you have a from time and a to time and the to time is smaller, then you can either interpret that as a negative range, or as a positive range crossing midnight. As I understand it, TimeOnly always chooses the latter option, which is why - is always going to return a positive value.

The result is a TimeSpan, because that is the correct type: TimeOnly represents a time of day (e.g. midnight), whereas TimeSpan represents a duration.