In subscription numbering patterns, one can set Koha to sub in season, month, or day of week in place of a number. But those substitutions all start with zero (so for months, 0=January, 1=February, and so on). This adds extra confusion to what's already an complicated module. We should make these values start at 1, as human counting works.
*** This bug has been marked as a duplicate of bug 28710 ***