How to validate that one datetime is greater than another

My class takes two datetimes startTime and endTime as input arguments. I would like to use argument validation to force endTime to be later than startTime, but mustBeGreaterThan does not accept datetimes.

Answer 1

[Stephen23]

Do NOT convert to low-precision serial date numbers! Avoid deprecated DATENUM !

Argument validation lets you define your own validation function, which makes your task easy (and you get to write exactly the informative error messages that you want to see displayed):

A = datetime(1994,1,1);
B = datetime(2024,1,1);
myfun(A,B)

okay!

myfun(B,A)

Error using solution>myfun (line 9)
Invalid argument at position 2. End time must be after start time.

function myfun(startTime,endTime)
arguments
    startTime (1,1) {idt}
    endTime (1,1) {idt,ilt(startTime,endTime)}
end
disp('okay!')
end
function idt(x)
assert(isdatetime(x),'Input must be a DATETIME.')
end
function ilt(s,e)
assert(s<e,'End time must be after start time.')
end

[Fangjun Jiang]

Thank you, @Stephen23!

[Rich006]

Aha! I had no idea I could write my own validation function. That will work perfectly. That said, a builtin mustBeLaterThan would surely be useful to a lot of developers.

[Rich006]

Is there a trick to make this work for validating arguments to a class constructor? When I try to instantiate the class below, I get error Incorrect number or types of inputs or outputs for function idt.

classdef myClass < handle
    methods
        function obj = myClass(startTime,endTime)
            arguments
                startTime (1,1) {idt}
                endTime (1,1) {idt,ilt(startTime,endTime)}
            end
            disp('okay!')
        end
        function idt(x)
            assert(isdatetime(x),'Input must be a DATETIME.')
        end
        function ilt(s,e)
            assert(s<e,'End time must be after start time.')
        end
    end % methods
end % classdef

[Stephen23]

Some changes:

• move the validation functions to local functions (rather than methods),
• do not use the same name for the method and the class,
• define the class object as a method input. This requirement is explained here:

https://www.mathworks.com/help/matlab/matlab_oop/class-components.html#bupsck8

https://www.mathworks.com/help/matlab/matlab_oop/specifying-methods-and-functions.html

After that (see attached file), it works as expected:

A = datetime(1994,1,1);
B = datetime(2024,1,1);

obj = myClass();
obj.myMethod(A,B)

okay!

obj.myMethod(B,A)

Error using myClass/myMethod (line 7)
Invalid argument at position 3. End time must be after start time.

[Rich006]

That still isn't what I was trying to do, because it requires running a method after construction. But I did use your suggestion of standalone validation functions. This allows me to create my own mustBeLater. And that answers my question!

classdef myClass
    properties
        startTime
        endTime
    end
    methods
        function obj = myClass(s,e)
        arguments
            s datetime
            e datetime {mustBeLater(e,s)}
        end
        obj.startTime = s;
        obj.endTime = e;
        disp('Success!')
        end
    end
end

function mustBeLater(laterTime,earlierTime)
if laterTime < earlierTime
    error("End time must be after start time.")
end
end

>> A = datetime('yesterday');
>> B = datetime('now');
>> obj = myClass(A,B);
Success!
>> obj = myClass(B,A);
Error using myClass
obj = myClass(B,A);
                ↑
Invalid argument at position 2. End time must be after start time.

(sorry, not sure how to format the inline command inputs/outputs)

Answer 2

[Joshua Levin Kurniawan]

You can use traditional logical operator for datatime. Operator "<", if true, means that the first datatime will occurs sooner than the next variable. In this case, just use the following code

if startTime < endTime
    disp("The data is valid");
else
    disp("The data is invalid");
end

For further information, you can check out the following page

https://www.mathworks.com/help/matlab/matlab_prog/compare-dates-and-time.html

[Rich006]

I'm trying to do this in argument validation, which is why I was trying to use mustBeGreaterThan.

Answer 3

[Fangjun Jiang]

It seems fine in R2022b, R2023b and R2024a

a=now;
b=now;
mustBeGreaterThan(b,a)
mustBeGreaterThan(a,b)

Value must be greater than 739389.6563.

[Rich006]

It works with numbers, but I'm trying to compare datetime objects.

[Fangjun Jiang]

okay, I thought now() returns a datetime object. It was not. You can convert it to datenum()

a=datetime('now');
b=datetime('now');
mustBeGreaterThan(datenum(b),datenum(a))
mustBeGreaterThan(datenum(a),datenum(b))

Value must be greater than 739389.6782.