public static boolean isInternalDateFormat(short format) {
        switch (format) {
            // Internal Date Formats as described on page 427 in
            // Microsoft Excel Dev's Kit...
            // 14-22
            case 0x0e:
            case 0x0f:
            case 0x10:
            case 0x11:
            case 0x12:
            case 0x13:
            case 0x14:
            case 0x15:
            case 0x16:
            // 45-47
            case 0x2d:
            case 0x2e:
            case 0x2f:
                return true;
        }
        return false;
    }
Given a format ID this will check whether the format represents an internal excel date format or not.

@see #isADateFormat(Short, String)
fastexcel-core/src/main/java/cn/idev/excel/util/DateUtils.java