day

Day number or name of input date and time

Syntax

d = day(t)

d = day(t,dayType)

Description

d = day(t) returns the day for each date and time in the input array. By default, day returns the day-of-month numbers for the elements of the input datetime array t.

The output d is a double array containing integer values from 1 to 31, depending on the month and year. To assign day values to datetime array t, use t.Day and modify the Day property.

d = day(t,dayType) returns the type of day number or name of datetime array t, specified as either "dayofmonth", "dayofweek", "dayofyear", "name", or "shortname".

Examples

Extract Day Number from Dates

Extract the day of month numbers from an array of dates.

t = [datetime('yesterday');datetime('today');datetime('tomorrow')]

t = 3×1 datetime
   08-Aug-2025
   09-Aug-2025
   10-Aug-2025

d = day(t)

d = 3×1

     8
     9
    10

Determine Day of Week

Determine the day of the week for an arbitrary date, by specifying 'name' as the second input to the day function.

t = datetime(2014,05,16)

t = datetime
   16-May-2014

d = day(t,'name')

d = 1×1 cell array
    {'Friday'}

Alternatively, specify 'dayofweek' to return the day of the week as a number.

d = day(t,'dayofweek')

d = 
6

Input Arguments

t — Input date and time

datetime array

Input date and time, specified as a datetime array.

To support existing code that previously required Financial Toolbox™, day also accepts serial date numbers and text as inputs, but they are not recommended. For more information, see Version History.

dayType — Type of day values

'dayofmonth' (default) | 'dayofweek' | 'iso-dayofweek' | 'dayofyear' | 'name' | 'shortname'

Type of day values, specified as a value in the table.

Value of dayType	Description
'dayofmonth' (default)	Day-of-month number, from 1 to 28, 29, 30, or 31. The range depends on the month.
'dayofweek'	Day-of-week number, from 1 to 7, where day 1 of the week is Sunday.
'iso-dayofweek' (since R2023a)	Day-of-week number, from 1 to 7, according to the ISO 8601 standard, where day 1 of the week is Monday.
'dayofyear'	Day-of-year number, from 1 to 365 or 366, depending on the year.
'name'	Full day names, for example, Sunday. For NaT datetime values, the day name is the empty character vector, ''.
'shortname'	Abbreviated day names, for example, Sun. For NaT datetime values, the day name is the empty character vector, ''.

Note

day returns day names in the language specified by the Locale option in the Datetime format section of the Settings window. To change the default datetime locale, see Modify Command Window Settings.

This argument is valid only when t is a datetime array.

Output Arguments

d — Day number or name

double array | cell array of character vectors

Day number or name, returned as a numeric array of integers of type double, or a cell array of character vectors. d is the same size as t.

Extended Capabilities

Tall Arrays

Calculate with arrays that have more rows than fit in memory.

The day function fully supports tall arrays. For more information, see Tall Arrays.

Thread-Based Environment

Run code in the background using MATLAB® backgroundPool or accelerate code with Parallel Computing Toolbox™ ThreadPool.

This function fully supports thread-based environments. For more information, see Run MATLAB Functions in Thread-Based Environment.

Distributed Arrays

Partition large arrays across the combined memory of your cluster using Parallel Computing Toolbox™.

This function fully supports distributed arrays. For more information, see Run MATLAB Functions with Distributed Arrays (Parallel Computing Toolbox).

Version History

Introduced in R2014b

R2023a: Return ISO day of week

To return the ISO day-of-week number, use the 'iso-dayofweek' option. In the ISO 8601 standard, Monday is day 1 of the week.

R2022a: Serial date number and text inputs not recommended

Although day supports serial date number and text inputs, datetime values are recommended instead. The datetime data type provides flexible date and time formats, storage out to nanosecond precision, and properties to account for time zones and daylight saving time.

To convert serial date numbers or text to datetime values, use the datetime function. For example:

t = datetime(738427.656845093,"ConvertFrom","datenum");
d = day(t)

d =

    28

There are no plans to remove support for serial date number or text inputs.

R2022a: day functions from Financial Toolbox and MATLAB combined

In previous releases, there were two day functions: one in MATLAB^® that accepted datetime values, and one in Financial Toolbox that accepted serial date numbers or text values. As of R2022a, you can call day without Financial Toolbox. When you call day from MATLAB, it accepts all previous input types.

For example, you can make these calls with a serial date number and text from MATLAB:

d = day(738427.656845093)

d =

    28

d = day("28-Sep-2021")

d =

    28

In addition, day accepts a second input for the date format when the first input is not a datetime array:

d = day(t,F)

In this syntax, specify F using a valid date format as documented for the datestr function, though 'Q' format specifiers are not supported. For example:

d = day("2021/28/09","yyyy/dd/mm")

d =

    28