Why between(datetime1, datetime2) is different from (-1)*between(datetime2, datetime1)?

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Haris K. 2024-3-4

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评论： Haris K. 2024-3-5

I am trying to measure the number of 'quarters' apart, between a single datetime r, and a vector of datetimes v. Could somebody please explain why Q1 & Q2 are different?

r = datetime(1991,6,30,'Format','dd/MM/yyyy'); 
v = dateshift(datetime(1990,12,31,'Format','dd/MM/yyyy'),'end','quarter',0:5);
%CASE 1
dif = between(r, v, 'quarters')
dif = 1×6 calendarDuration array
   -1q    0q    0q    1q    2q    3q
Q1 = split(dif, 'quarters')
Q1 = 1×6
    -1     0     0     1     2     3
%CASE 2
dif = between(v, r, 'quarters')
dif = 1×6 calendarDuration array
    2q    1q    0q   -1q   -2q   -3q
Q2 = (-1)*split(dif, 'quarters')
Q2 = 1×6
    -2    -1     0     1     2     3

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the cyclist 2024-3-4

编辑：the cyclist 2024-3-4

在 MATLAB Online 中打开

A distilled version of the question is, why is there a "magnitude" difference between the outputs below? It is admittedly counter-intuitive.

dt1 = datetime("30/06/1991","Format",'dd/MM/yyyy');
dt2 = datetime("31/12/1990","Format",'dd/MM/yyyy');
between(dt1,dt2)
ans = calendarDuration
   -5mo -30d
between(dt2,dt1)
ans = calendarDuration
   6mo

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Answer 1

Voss 2024-3-4

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https://ww2.mathworks.cn/matlabcentral/answers/2090281-why-between-datetime1-datetime2-is-different-from-1-between-datetime2-datetime1#answer_1421251

移动：Dyuman Joshi 2024-3-5

From the between documentation:

"In general, t2 is not equal to t1 + dt, unless you include 'time' in components."

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Dyuman Joshi 2024-3-5

在 MATLAB Online 中打开

r = datetime(1991,6,30,'Format','dd/MM/yyyy')
r = datetime
   30/06/1991
v = dateshift(datetime(1990,12,31,'Format','dd/MM/yyyy'),'end','quarter',0:5)
v = 1×6 datetime array
   31/12/1990   31/03/1991   30/06/1991   30/09/1991   31/12/1991   31/03/1992

The difference is not symmetric/identical for every time component -

str = {'years','quarters','months','weeks','days' };
for k=1:numel(str)
    disp(str{k})
    disp('CASE 1')
    disp(between(r, v, str{k}))
    disp('CASE 2')
    disp(between(v, r, str{k}))
    disp(' ')
end
years
CASE 1
   0d   0d   0d   0d   0d   0d
CASE 2
   0d   0d   0d   0d   0d   0d
 
quarters
CASE 1
   -1q    0q    0q    1q    2q    3q
CASE 2
    2q    1q    0q   -1q   -2q   -3q
 
months
CASE 1
   -5mo   -2mo    0mo    3mo    6mo    9mo
CASE 2
    6mo    3mo    0mo   -3mo   -6mo   -9mo
 
weeks
CASE 1
   -25w   -13w     0w    13w    26w    39w
CASE 2
    25w    13w     0w   -13w   -26w   -39w
 
days
CASE 1
   -181d    -91d      0d     92d    184d    275d
CASE 2
    181d     91d      0d    -92d   -184d   -275d
 

Haris K. 2024-3-5

Thank you. The comment in the documentation is clearly understood. What I do not understand is why the order of passing inputs t1 and t2, should alter the result.

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