Subtract DateOnly in C#

Question

In C# I can't use subtraction with DateOnly variables, unlike DateTime. Is there any explanation?

  var a = new DateTime(2000, 01, 01);
  var b = new DateTime(1999, 01, 01);

  //var c = a.Subtract(b);
  var c = a - b;

  var d = new DateOnly(2000, 01, 01);
  var e = new DateOnly(1999, 01, 01);

  var f = d - e; // Error - Operator '-' cannot be applied to operands of type 'DateOnly' and 'DateOnly'

Adolphadolphe

Does this answer your question? Calculate difference between two dates (number of days)? – Foreshow

You can use extension .ToDateTime(bool timeOnly) on your DateOnly so you can have behaviour of DateTime – Nearly

@Amit Verma Thanks for answer, but not really. I need to use DateOnly structure instead of DateTime, which is used in link. – Adolphadolphe

Since you aren't asking for a solution i figure my answer will be sufficient? – Ranit

When it was introduced: "a DateOnly represents the entire date (from the start of the day through the end of the day)". Subtracting one of those from another would be problematic because there are 3 possible answers depending on what inclusivity you would want to consider for the start and end points. – Selfrestraint

@rbdeebk thanks for answer! I'm more interested in the reason – Adolphadolphe

Answer 1

Conceptually DateOnly represents an entire day, not midnight or any other specific time on a given day, such that subtracting one DateOnly from another cannot logically return a TimeSpan as with DateTime's subtraction operator.

If you want to perform arithmetic on DateOnlys, you need to be explicit about the desired unit.

DateOnly has a DayNumber property, that returns the number of whole days since 01/01/0001, so if you want to determine the number of whole days between 2 DateOnly values, you can do the following:

var d = new DateOnly(2000, 01, 01);
var e = new DateOnly(1999, 01, 01);

var daysDifference = d.DayNumber - e.DayNumber;

Yost

Hi, If we suppose, one of them is nullable how can I do that? – Cf

@L.Kvri Use .Value or cast to DateOnly, along with any necessary null checks, as with other nullable value types. – Yost

Answer 2

You can use DayNumber property to do the subtraction, f will hold the number of days between d and e.

var f = d.DayNumber - e.DayNumber;

Kawasaki

Answer 3

Consider this: June 2, 2022 - June 1, 2022 = ? Is the answer 1 or is the answer 2? It encompasses 2 full days, but the difference is 1. By forcing us to use DayNumber, there is only one possible answer.

Led

The answer is 1 :-) – Adust

It's really simple, June 1 minus June 1 is obviously zero. By forcing us to use DayNumber exactly nothing is made any less error-prone, just a bit more typing to do... – Yawn

Answer 4

To answer your question why - DateOnly in operators section it just doesn't have subtraction implemented while DateTime does have one. What you can do is create your own extension similar to this one.

Electuary