Current Behavior

Grouping by month allows ambiguous years such as XXXX-01-XX.

Expected behavior

Strains with ambiguous years should be dropped since month is supposed to represent the exact month in time YYYY-MM, not just the month number MM.

How to reproduce

cat >metadata.tsv <<~~
strain	date
SEQ1	1XXX-01-01
SEQ2	2XXX-01-01
SEQ3	3XXX-01-01
~~

augur filter \
   --metadata metadata.tsv \
   --group-by month \
   --sequences-per-group 1 \
   --subsample-seed 0 \
   --output-metadata out.tsv

cat out.tsv
# strain	date
# SEQ1	1XXX-01-01

Possible solution

Check for ambiguous years when grouping by month.