a date_xx or any R object that can be coerced to POSIXlt
label
logical. TRUE will display the month as a character string such
as "January." FALSE will display the month as a number.
abbr
logical. FALSE will display the month as a character string
label, such as "January". TRUE will display an abbreviated version of the
label, such as "Jan". abbr is disregarded if label = FALSE.
locale
for month, locale to use for month names. Default to current locale.