If-Modified-Since = "If-Modified-Since" ":" HTTP-date 以上、RFC2616 14.25 If-Modified-Since より HTTP-date = rfc1123-date | rfc850-date | asctime-date rfc1123-date = wkday "," SP date1 SP time SP "GMT" rfc850-date = weekday "," SP date2 SP time SP "GMT" asctime-date = wkday SP date3 SP time SP 4DIGIT date1 = 2DIGIT SP month SP 4DIGIT ; day month year (e.g., 02 Jun 1982) date2 = 2DIGIT "-" month "-" 2DI