if no instrument can be found by the id of x, or if the 
instrument does not have an expires attribute, an attempt
will be made to infer the year and month of expiration using parse_id
in which case the returned value will be a string of the format 
“YYYY-MM”.  Presently, Date and expired will be ignored 
if x is not the name of an instrument
# S3 method for character
expires(x, Date, expired = TRUE, silent = FALSE, ...)Can be a Date or character string.  When expires is a 
vector, the retuned value will be one of the two values of expires 
that are closest to Date. (which one will be determined by the value 
of expired).
TRUE/FALSE. This determines which date will be used when
expires is a vector.  If expired is TRUE the date 
returned will be the last one before Date.  If expired is 
FALSE the first one after Date will be returned.
silence warnings?