Odd behavior using datetime

I am using the datetime function to convert numbers into dates. For example:

datetime(num2str(2072016),'InputFormat','ddMMyyyy')

ans = datetime
   02-Jul-2016

This works as expected, but -for some unknown reason to me- this fails on certain dates, e.g.:

datetime(num2str(4072016),'InputFormat','ddMMyyyy')

Error using datetime (line 667)
Unable to convert '4072016' to datetime using the format 'ddMMyyyy'.

Why does this happen? Am I doing something wrong? Is this a bug?

However, if I only specify one 'd' on my InputFormat instead of 2, this works as expected:

datetime(num2str(4072016),'InputFormat','dMMyyyy')

I am aware of other ways to input numbers to use the datetime function, but in my case this is the way that takes the less number of lines in my code. Also, a workaround for this should not be necessary since this way of converting a string into a date should work regardless of the date in question.

My MATLAB version is 2024a. Although I do not know whether this is a thing in other versions as well.

Thanks!

Answer 1

[dpb]

I'm surprised the first works; it shouldn't either.

See the doc for datetime <time formats>. As it notes,

d     Day of the month, using one or two digits
dd    Day of the month using two digits

The date strings should be

datetime('04072016','InputFormat','ddMMyyyy')

ans = datetime
   04-Jul-2016

to use 'dd'.

If you do not want to incorporate the leading zero on days, then you want/need the single-letter date format string. Note, by the way, the same rule holds for months, 'M' and 'MM' and the various time fields as well. Read all of the documentation!!! :)

Also note that neither

num2str(04072016)

ans = '4072016'

string(04072016)

ans = "4072016"

keep the nonsignificant leading zero if you are reading date strings in as numeric; you would need a format string to convert them to include such.

ADDENDUM/ERRATUM:

While the datetime documentation format description describes that the single-letter form will accept either one- or two-digit days/months, datetime seems unable to convert the two-digit form given the single digit format. I think this is either a bug or the documentation should be corrected to reflect actual behavior. The description section of the documentation using 'infmt' states that all strings must be the same format, but the input array are all compliant with the one- or two- character and the input format string provided that is two-digits for month and four for year.

Answer 2

[Star Strider]

If there are more than one value that you want to convert, perhaps something like this —

date = [4072016 12072016];
sdate = cellfun(@num2str, num2cell(date), 'Unif',0)

sdate = 1x2 cell array
    {'4072016'}    {'12072016'}

for k = 1:size(sdate,2)                                 % Zero-Pad 'date' Values With Odd Numbers Of Characters
    if rem(numel(sdate{k}),2) ~= 0
        sdate{k} = ['0' sdate{k}];
    end
end

Out = datetime(sdate.', InputFormat="ddMMyyyy")

Out = 2x1 datetime array
   04-Jul-2016
   12-Jul-2016

Any leading zeros would be left off of a date specified only as a number.

.

[dpb]

dates = string([4072016 2072016 12072016]).'

dates = 3x1 string array
    "4072016"
    "2072016"
    "12072016"

dt=datetime(dates, InputFormat="dMMyyyy")

dt = 3x1 datetime array
   04-Jul-2016
   02-Jul-2016
   NaT        

dt=arrayfun(@(s)datetime(s, InputFormat="dMMyyyy"),dates)

Error using datetime (line 667)
Unable to convert '12072016' to datetime using the format 'dMMyyyy'.

Error in solution>@(s)datetime(s,InputFormat="dMMyyyy") (line 3)
dt=arrayfun(@(s)datetime(s, InputFormat="dMMyyyy"),dates)

Looks to me as though that uncovers a bug in that the documentation claims the 'd' form of the day field should accept either one- or two-digit months and certainly all of the other fields would match the input format specification of two-digit month and four-digit year and are valid dates.

ADDENDUM:

I suppose the following caveat in the input documentation is the out .. "All values in the input argument DateStrings must have the same format.". It doesn't really make sense then to be able to specify the one-letter version as being able to do both if it can actually only do one or the other in any given call altough now that am reminded I now recall having gone through this at least once before.

An alternative approach with newer string array functionality to pad the strings is given as an alternative Answer...

[Stephen23]

vec = [4072016 12072016];
str = compose('%08i',vec(:))

str = 2x1 cell array
    {'04072016'}
    {'12072016'}

[dpb]

Another good alternative, @Stephen23!

vec = [4072016 12072016];
str = num2str(vec(:),'%08i')

str = 2x8 char array
    '04072016'
    '12072016'

also works, but returns a char() array unless cast to cellstr for datetime()

The arithmetic to convert to y,m,d arrays is not bad, either

y=rem(vec,10000);
m=fix(rem(vec/10000,100));
d=fix(rem(vec/1000000,100));
datetime(y,m,d).'

ans = 2x1 datetime array
   04-Jul-2016
   12-Jul-2016

Answer 3

[dpb]

Have a better mousetrap, I think...

dates = string([4072016 2072016 12072016]).';
dt=datetime(pad(dates,8,'left','0'),'InputFormat','ddMMyyyy')

dt = 3x1 datetime array
   04-Jul-2016
   02-Jul-2016
   12-Jul-2016

using the new(ish) strings class functionality to pad the shorter rows -- same end result as @Star Strider's with the explicit code, of course.

BTW, I'd caution against using date as a variable, it is a MATLAB-supplied function that gets aliased and might be confusing later in code dealing with time and dates...

Answer 4

[dpb]

Another alternative without converting the numeric values to strings...

dates = [4072016 2072016 12072016].';

y=rem(dates,10000);
m=fix(rem(dates/10000,100));
d=fix(rem(dates/1000000,100));
datetime(y,m,d)

ans = 3x1 datetime array
   04-Jul-2016
   02-Jul-2016
   12-Jul-2016

Or, the date vector can be passed if more convenient

ymd=[y m d];
datetime(ymd)

ans = 3x1 datetime array
   04-Jul-2016
   02-Jul-2016
   12-Jul-2016