The next element of 
fmt is equal to 
'%',
optionally followed by a modifier character,
followed by a conversion specifier character, 
format,
together forming a conversion specification
valid for the POSIX function 
strptime.If the number of elements in the range [
fmt, fmtend)
is not sufficient to unambiguously determine
whether the conversion specification is complete and valid,
the function evaluates 
err = ios_base::failbit.Otherwise,
the function evaluates 
s = do_get(s, end, f, err, t, format, modifier),
where the value of 
modifier is 
'\0'
when the optional modifier is absent from the conversion specification
.If 
err == ios_base::goodbit holds
after the evaluation of the expression,
the function increments 
fmt
to point just past the end of the conversion specification and
continues looping
.