With biber 2.6 comes a unnecassary warning:
[15251] Utils.pm:164> WARN - month field 'Apr' in entry 'GSP+08' is not an integer - this will probably not sort properly.
Is the three letter month code no longer supported? the warning clatters the log.
year/monthfields - thedatefield has many more features andyear/monthare considered legacy inbibereven though they are still supported. – PLK Sep 18 '16 at 13:59