private Format createDateFormat()

in poi/src/main/java/org/apache/poi/ss/usermodel/DataFormatter.java [564:704]
120 lines of code
35 McCabe index (conditional complexity)

    private Format createDateFormat(String pFormatStr, double cellValue) {
        String formatStr = adjustTo4DigitYearsIfConfigured(pFormatStr);
        formatStr = formatStr.replace("\\-","-");
        formatStr = formatStr.replace("\\,",",");
        formatStr = formatStr.replace("\\.","."); // . is a special regexp char
        formatStr = formatStr.replace("\\ "," ");
        formatStr = formatStr.replace("\\/","/"); // weird: m\\/d\\/yyyy
        formatStr = formatStr.replace(";@", "");
        formatStr = formatStr.replace("\"/\"", "/"); // "/" is escaped for no reason in: mm"/"dd"/"yyyy
        formatStr = formatStr.replace("\"\"", "'"); // replace Excel quoting with Java style quoting
        formatStr = formatStr.replace("\\T","'T'"); // Quote the T is iso8601 style dates


        boolean hasAmPm = false;
        Matcher amPmMatcher = amPmPattern.matcher(formatStr);
        while (amPmMatcher.find()) {
            formatStr = amPmMatcher.replaceAll("@");
            hasAmPm = true;
            amPmMatcher = amPmPattern.matcher(formatStr);
        }
        formatStr = formatStr.replace('@', 'a');


        Matcher dateMatcher = daysAsText.matcher(formatStr);
        if (dateMatcher.find()) {
            String match = dateMatcher.group(0).toUpperCase(Locale.ROOT).replace('D', 'E');
            formatStr = dateMatcher.replaceAll(match);
        }

        // Convert excel date format to SimpleDateFormat.
        // Excel uses lower and upper case 'm' for both minutes and months.
        // From Excel help:
        /*
            The "m" or "mm" code must appear immediately after the "h" or"hh"
            code or immediately before the "ss" code; otherwise, Microsoft
            Excel displays the month instead of minutes."
          */

        StringBuilder sb = new StringBuilder();
        char[] chars = formatStr.toCharArray();
        boolean mIsMonth = true;
        List<Integer> ms = new ArrayList<>();
        boolean isElapsed = false;
        for(int j=0; j<chars.length; j++) {
            char c = chars[j];
            if (c == '\'') {
                sb.append(c);
                j++;

                // skip until the next quote
                while(j<chars.length) {
                    c = chars[j];
                    sb.append(c);
                    if(c == '\'') {
                        break;
                    }
                    j++;
                }
            }
            else if (c == '[' && !isElapsed) {
                isElapsed = true;
                mIsMonth = false;
                sb.append(c);
            }
            else if (c == ']' && isElapsed) {
                isElapsed = false;
                sb.append(c);
            }
            else if (isElapsed) {
            if (c == 'h' || c == 'H') {
                    sb.append('H');
                }
                else if (c == 'm' || c == 'M') {
                    sb.append('m');
                }
                else if (c == 's' || c == 'S') {
                    sb.append('s');
                }
                else {
                    sb.append(c);
                }
            }
            else if (c == 'h' || c == 'H') {
                mIsMonth = false;
                if (hasAmPm) {
                    sb.append('h');
                } else {
                    sb.append('H');
                }
            }
            else if (c == 'm' || c == 'M') {
                if(mIsMonth) {
                    sb.append('M');
                    ms.add(sb.length() - 1);
                } else {
                    sb.append('m');
                }
            }
            else if (c == 's' || c == 'S') {
                sb.append('s');
                // if 'M' precedes 's' it should be minutes ('m')
                for (int index : ms) {
                    if (sb.charAt(index) == 'M') {
                        sb.replace(index, index + 1, "m");
                    }
                }
                mIsMonth = true;
                ms.clear();
            }
            else if (Character.isLetter(c)) {
                mIsMonth = true;
                ms.clear();
                if (c == 'y' || c == 'Y') {
                    sb.append('y');
                }
                else if (c == 'd' || c == 'D') {
                    sb.append('d');
                }
                else {
                    sb.append(c);
                }
            }
            else {
                if (Character.isWhitespace(c)){
                    ms.clear();
                }
                sb.append(c);
            }
        }
        formatStr = sb.toString();

        try {
            return new ExcelStyleDateFormatter(formatStr, dateSymbols);
        } catch(IllegalArgumentException iae) {
            LOG.atDebug().withThrowable(iae).log("Formatting failed for format {}, falling back", formatStr);
            // the pattern could not be parsed correctly,
            // so fall back to the default number format
            return getDefaultFormat(cellValue);
        }

    }