`day` — Extract day-of-month numbers from datetime values in MATLAB and RunMat.

day extracts day-of-month values from datetime scalars or arrays and returns doubles with matching shape, consistent with MATLAB behavior.

Syntax

X = day(t)

Inputs

Name	Type	Required	Default	Description
`value`	`Any`	Yes	—	Datetime input.

Returns

Name	Type	Description
`X`	`Any`	Numeric scalar/tensor result.

Errors

Identifier	When	Message
`RunMat:datetime:InvalidArgument`	Arguments or option grammar do not match supported datetime forms.	`datetime: invalid argument`
`RunMat:datetime:InvalidInput`	Input values cannot be parsed/converted/broadcast to a valid datetime result.	`datetime: invalid input`
`RunMat:datetime:Internal`	Internal datetime state or indexing/evaluation failed unexpectedly.	`datetime: internal operation failed`

How `day` works

day(t) accepts datetime scalars and datetime arrays.
The output shape matches the input shape.
Day values reflect the calendar day stored in the datetime object.
Passing a non-datetime value raises a datetime-specific error.

Does RunMat run `day` on the GPU?

day does not dispatch GPU kernels; it extracts the day component from CPU-side datetime objects.

GPU memory and residency

No. day reads datetime objects on the CPU and returns ordinary host-side numeric arrays.

Examples

Extract the day from a scalar datetime

t = datetime(2024, 4, 9, 13, 30, 0);
d = day(t)

Expected output:

d = 9

Extract days from a datetime array

t = datetime([2024 2025], [1 6], [15 20]);
d = day(t)

Expected output:

d =
    15    20

Using `day` with coding agents

Open a RunMat example with live inputs, then ask the agent to explain how day changes the result.

Run a small day example, explain the result, then change one input and compare the output.

FAQ

What day does day return?⌄

It returns the calendar day-of-month, such as 9 for April 9.

Does day preserve the input shape?⌄

Yes. The numeric output has the same size as the input datetime array.

Does day run on the GPU?⌄

No. datetime values are host-side objects, so day executes on the CPU.

datetime · duration · hour · minute · month · second · year

Open-source implementation

Unlike proprietary runtimes, every RunMat function is open-source. Read exactly how day is executed, line by line, in Rust.

View the source for day in Rust on GitHub
Learn how the RunMat runtime works
Found a bug? Open an issue with a minimal reproduction.

About RunMat

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day — Extract day-of-month numbers from datetime values in MATLAB and RunMat.