[threeten-dev] incompatibility issues between MinguoDate.minus(Period) and MinguoDate.periodUntil(MinguoDate)

[patrick zhang]

Hi Team,

As you know, MinguoDate.minus(Period) will return one new MinguoDate 
while MinguoDate.periodUntil(MinguoDate) will calculate the period 
between 2 date.
so I think:
if
    minguodate2 = minguodate1.minus(period) ;
then
    minguodate2.periodUntil(minguodate1) = period.

I write sample code to check it. It looks sometime it works but 
sometimes it fails.
You can see date3 and date4 work as expected while data1 and date2 look 
strange.

I know it is caused by different length of months. And Period class will 
do something to match it. 
But I am really a bit confused by the output. Is it correct behavior?

Code:
=============================
         MinguoDate date1 = MinguoDate.of(1970, 1, 1) ;
        Period p = Period.of(1, 1, 1) ;
        MinguoDate date2 = date1.minus(p) ;   
       
        System.out.println(p) ;
        System.out.println(date2.periodUntil(date1)) ;
       
        System.out.println("**************") ;
        MinguoDate date3 = MinguoDate.of(1970, 5, 3) ;
        MinguoDate date4 = date3.minus(p) ;   
       
        System.out.println(p) ;
        System.out.println(date4.periodUntil(date3)) ;
===============================

Output:
===============================
P1Y1M1D
P1Y1M2D
**************
P1Y1M1D
P1Y1M1D
===============================

Regards
Patrick