Convert a datetime to string

I want to convert a specific datetime from a table into a string with a certain format, which it also uses in the table. I tried using datestring, with a format for the output : checkdate = datestr(datelist{1,d},'yyyy_MM_dd')

When I use this format for the table I get: 2020_07_17

But when using it with the datestr I get this: '22-Feb-2022'. How can I get the same format from the table as a string?

[Walter Roberson]

Why are you using datestr at all? Why are you not using datetime objects?

[Stephen23]

Note that the DATESTR documentation states "(Not recommended; use string or char)"

You should use STRING or CHAR.

[Lieke van Boxtel]

I need to use the dates to look up if some files exist and then read those that exist. This is why I need to make them strings, to be able to add them to the filename.

[Walter Roberson]

@Lieke van Boxtel

The datestr call does produce character vectors based upon a variety of ways of inputting times. However, it is based on an older Mathworks time implementation that Mathworks calls "serial date numbers", which are "number of days and fraction of days since the beginning of year 0 CE." For example,

format long g;
now

ans = 
          739213.334071768

so were are currently approaching 3/4 of a million days since the beginning of 0 CE.

The time resolution available with serial date numbers is roughly 10 microseconds.

This is not the only way that MATLAB has of representing time. Mathworks added what it calls "datetime objects" and "duration objects". Datetime objects internally use two double precision objects together, datetime objects have a resolution better than seconds.

If you have a datetime object and you want to convert it to a character vector with a particular format, first set the Format property of the object to the format you want, and then char() or string() the datetime object.

extime = datetime('July 17, 2020 15:43:19')

extime = datetime
   17-Jul-2020 15:43:19

string(extime)

ans = "17-Jul-2020 15:43:19"

extime.Format = 'yyyy_MM_dd';
string(extime)

ans = "2020_07_17"

[Stephen23]

"I want to convert a specific datetime from a table ... But when using it with the datestr I get this: '22-Feb-2022'"

dt = datetime(2022,2,22) % curiously many 2's

dt = datetime
   22-Feb-2022

datestr(dt,'yyyy_MM_dd') % does not match what you say you got

ans = '2022_00_22'

If you used DATESTR with that format then there is no way you could get a month abbreviation from uppercase M's.

If you used the correct format (as Alexander Paul showed) then you could:

datestr(dt,'yyyy_mmm_dd') % does not match what you say you did

ans = '2022_Feb_22'

"When I use this format for the table I get: 2020_07_17"

If the data in the table is not really a DATETIME but really text then note that underscores are not part of any of the supported syntaxes for input date strings. Lets see what happens anyway:

datestr('2022_07_17','yyyy_mmm_dd')

ans = '2022_Jul_17'

Well, DATESTR seems to have coped with that just fine. Without uploading sample data it is unclear what the problem is.

Answer 1

[Star Strider]

I am not certain what the problem is.

To convert [2022 07 17] to your desired format using datetime, try this —

DT = datetime([2022 07 17], 'Format','yyyy_MM_dd')

DT = datetime
   2022_07_17

To convert it to a string variable:

DTstr = string(DT)

DTstr = "2022_07_17"

and to a character array:

DTchr = sprintf('%s', DT)

DTchr = '2022_07_17'

.

[Lieke van Boxtel]

Thanks, doing seperate steps using string did the trick for me.

[Star Strider]

As always, my pleasure!

Answer 2

[Walter Roberson]

if you have a datetime object then set its Format property. In some cases that is all you need to do; in other cases you need to string() or char() the datetime object afterwards.

Answer 3

[Alexander]

Use mm instead of MM.

checkdate = datestr(datelist{1,d},'yyyy_mm_dd')

should do the work. As I have not your datelist, I haven't tested it.

[Lieke van Boxtel]

The small mm is for minutes and the MM is for months.

[Stephen23]

"The small mm is for minutes and the MM is for months."

Not for DATESTR:

https://www.mathworks.com/help/matlab/ref/datestr.html#btenptl-1-formatOut

Alexander Paul is quite correct. You are likely confusing the format for DATETIME.

[Alexander]

You mixing up datestr and datetime! With datestr you have to use mm for months and MM for minutes.

https://de.mathworks.com/help/matlab/ref/datestr.html

https://de.mathworks.com/help/matlab/ref/datetime.html

[Lieke van Boxtel]

Ah, thanks, yes I was confusing those. Weird that it is different in this case, but good to know.