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

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

[the cyclist]

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

Answer 1

[Voss]

From the between documentation:

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

[Dyuman Joshi]

dt1 = datetime("30/06/1991","Format",'dd/MM/yyyy');
dt2 = datetime("31/12/1990","Format",'dd/MM/yyyy');

between(dt1,dt2,'time')

ans = calendarDuration
   -4344h 0m 0s

between(dt2,dt1,'time')

ans = calendarDuration
   4344h 0m 0s

[Dyuman Joshi]

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.]

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.