cell array with date and time

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babyelephant 2019-8-12

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https://ww2.mathworks.cn/matlabcentral/answers/475802-cell-array-with-date-and-time

评论： dpb 2019-8-14

{[11-Jun-1962 00:00:00]}
{[16-Mar-1961 00:00:00]}
{[02-Jun-1934 00:00:00]}
{[02-Jun-1934 00:00:00]}
{[09-Jun-1965 00:00:00]}
{[09-Jun-1965 00:00:00]}
{[13-Apr-1955 00:00:00]}
{[13-Apr-1955 00:00:00]}
{[09-Dec-1947 00:00:00]}

I have a cell array like above.

I want to convert this to a cell array only with dates.

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Steven Lord 2019-8-13

在 MATLAB Online 中打开

This is completely different behavior than the venerable (and deprecated) datenum which stored dates as the integer portion of a float and time as the fraction of days. Thus, one could "zero out" and keep only the integer or fractional portion. Not possible w/ datetime.

You're right that you can't do this with datetime alone. That's one of the reasons duration exists.

rightNow = datetime('now');
dateWithoutTime = dateshift(rightNow, 'Start', 'day')
timeSinceMidnight = rightNow - dateWithoutTime
rightNow == (dateWithoutTime + timeSinceMidnight) % true
whos rightNow dateWithoutTime timeSinceMidnight

You want to know how many days it's been since midnight, what you would receive from the fractional part of the serial date number? That's easy.

D = days(timeSinceMidnight)

Since it's just after 10:30 in the morning here in Natick as I type this, D is about 0.44. That's just about what I receive by taking mod of the serial date number created from rightNow using datenum on it and 1.

D2 = mod(datenum(rightNow), 1)

You may ask whether or not dateshift is discoverable easily. If you try calling floor on a datetime like you would on a serial date number to truncate it, the error you receive tells you about dateshift (at least in release R2019a, and I suspect in earlier releases as well.)

>> floor(rightNow)
Error using datetime/floor (line 1608)
Undefined function 'floor' for input arguments of type 'datetime'. Use the DATESHIFT function instead.

dpb 2019-8-13

编辑：dpb 2019-8-13

在 MATLAB Online 中打开

No argument aginst datetime being more powerful -- the comment was intended only to emphasize the difference as often see people who expect to be able to create a datetime without one or the other of the two components--particularly often the time of day without the date.

But, adding five calendar years isn't that simple with datetime() either... :)

>> t0=datetime(date);
>> t0+years(5)
ans = 
  datetime
   12-Aug-2024 05:06:00
>> 

isn't 13-Aug-2024 but something less with odd hours and minutes. years(N) when the argument is numeric returns in units of fixed-length, 365.2425 day years. It has no time reference from which to compute how many leapyears are included in the span so is just long-term average.

To get five "real" years, you have to use the calyears() function

>> t0+calyears(5)
ans = 
  datetime
   13-Aug-2024
>> 

which has enough additional logic built into the combination of it and the overloaded plus operator to do the calendar calculations.

OTOH, for this datenum is easy...

>> datestr(datenum(2019+5,8,13))
ans =
    '13-Aug-2024'
>> 

as it has the calendar arithmetic builtin and so just adding 5 to the year is all that is needed. Now, granted, to get the year from date() string means parsing it back out or using datevec() to extract the components of t0.

>> datestr(datenum(datevec(t0)+[5,0,0,0,0,0]))
ans =
    '13-Aug-2024'
>>

This diverged from the point of the comment which wasn't to denigrate datetime but just to emphasize that it is, as you note, a compound object, not just a number like a datenum is (albeit with a very specialized meaning/scaling).

dpb 2019-8-14

That was an inadvertent INT() for FIX() , Walter. Seems in context of the reply would have been clear what was intended. Often fall into that as while ML has the the many intXX() versions, there's no generic as used to in Fortran...

dpb 2019-8-14

在 MATLAB Online 中打开

>> datetime(2008+5,2,29)
ans = 
  datetime
   01-Mar-2013
>>

"+calyears()" is probably the more expected behavior for financial and date-keeping purposes.

>> which  -all plus
built-in (C:\ML_R2017\toolbox\matlab\ops\@double\plus)       % double method
built-in (C:\ML_R2017\toolbox\matlab\ops\@uint8\plus)        % uint8 method
built-in (C:\ML_R2017\toolbox\matlab\ops\@uint16\plus)       % uint16 method
...
built-in (C:\ML_R2017\toolbox\matlab\ops\@single\plus)       % single method
built-in (C:\ML_R2017\toolbox\matlab\ops\@char\plus)         % char method
built-in (C:\ML_R2017\toolbox\matlab\ops\@logical\plus)      % logical method
plus is a built-in method                                    % string method
C:\ML_R2017\toolbox\matlab\timefun\@duration\plus.m          % duration method
C:\ML_R2017\toolbox\matlab\timefun\@calendarDuration\plus.m  % calendarDuration method
C:\ML_R2017\toolbox\matlab\timefun\@datetime\plus.m          % datetime method
...
>> 

All three classes have methods specific to them.

The upshot is, one has to have an understanding of how things work to ensure one gets the result wanted for a particular purpose. Any of the above could conceivably be the desired result for a specific application.

The conversation has pointed out more specifics of the original point made...a datetime is a totally different animal than a datenum...

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