[Octave-bug-tracker] [bug #60671] Wrong result when subtracting one month from 31-MON-YEAR date

[anonymous]

Follow-up Comment #1, bug #60671 (project octave):

Thank you for the bug report. It looks like the behavior is chaotic about
whether it returns the same month or the previous month:


octave:12> datestr (addtodate (datenum('31-Jan-2020'), -1,'month'))
ans = 31-Dec-2019
octave:13> datestr (addtodate (datenum('31-Mar-2020'), -1,'month'))
ans = 02-Mar-2020
octave:14> datestr (addtodate (datenum('31-May-2020'), -1,'month'))
ans = 01-May-2020
octave:15> datestr (addtodate (datenum('31-Jul-2020'), -1,'month'))
ans = 01-Jul-2020
octave:16> datestr (addtodate (datenum('31-Aug-2020'), -1,'month'))
ans = 31-Jul-2020
octave:17> datestr (addtodate (datenum('31-Oct-2020'), -1,'month'))
ans = 01-Oct-2020
octave:18> datestr (addtodate (datenum('31-Dec-2020'), -1,'month'))
ans = 01-Dec-2020


For the other months, it at least seems to be consistently decrementing the
month:


octave:20> datestr(addtodate (datenum('29-Feb-2020'), -1,'month'))
ans = 29-Jan-2020
octave:21> datestr(addtodate (datenum('30-Apr-2020'), -1,'month'))
ans = 30-Mar-2020
octave:22> datestr(addtodate (datenum('30-Jun-2020'), -1,'month'))
ans = 30-May-2020
octave:23> datestr(addtodate (datenum('30-Sep-2020'), -1,'month'))
ans = 30-Aug-2020
octave:24> datestr(addtodate (datenum('30-Nov-2020'), -1,'month'))
ans = 30-Oct-2020


The bug seems to be in these two lines within addtodate.m, which fail
sometimes when the second argument q is negative:


      dtmp(:,1) += floor ((dtmp(:,2)-1)/12);
      dtmp(:,2) = mod (dtmp(:,2)-1, 12) + 1;



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