parseEnglish
Parses a date string into an English CustomCalendar, the counterpart of parse.
Return
a CustomCalendar, or null in case the parsing failed. A syntactically valid date that does not exist (April 31, say) comes back with totalDaysInMonth = -1 rather than as null, matching parse, so callers can tell "not a date" from "not a real day".
Parameters
dateString
a date string in the format "yyyyMMdd"