・指定した二つ日付の間の月を取得
　startFy：202009
　endFY：202109
・返却値：
　202009,202010,202011,202012,202101,202102,202103,202104,202105,202106,202107,202108,202109
String startFy = '202009';
String endFY= '202109';
// getFYを呼び出す
Set<String> setFY = getFY(startFy, endFY)

/*
 * 年月を取得
*/
public Set<String> getFY(String startFy, String endFY){
    Set<String> setFY = new Set<String>();
    
    if (String.isEmpty(startFY) || String.isEmpty(endFY)) {
        return setFY;
    }
    // 削除開始年月の取得
    Integer startYear = Integer.valueOf(startFY.substring(0, 4));
    Integer startMonth = Integer.valueOf(startFY.substring(4, 6));
    // 削除終了年月の取得
    Integer endYear = Integer.valueOf(endFY.substring(0, 4));
    Integer endMonth = Integer.valueOf(endFY.substring(4, 6));
    for (integer i = startYear; i <= endYear; i++) {
        String dateFy = '';
        if (startYear == endYear) {
            for (integer j = startMonth; j <= endMonth; j++) {
                if (j < 10) {
                    dateFy = i + '0' + j;
                } else {
                    dateFy = i + '' + j;
                }
                setFY.add(dateFy);
            }
        } else {
            if (i == startYear) {
                for (integer j = startMonth; j <= 12; j++) {
                    if (j < 10) {
                        dateFy = i + '0' + j;
                    } else {
                        dateFy = i + '' + j;
                    }
                    setFY.add(dateFy);
                } 
            } else if (i == endYear) {
                for (integer j = 1; j <= endMonth; j++) {
                    if (j < 10) {
                        dateFy = i + '0' + j;
                    } else {
                        dateFy = i + '' + (j + 1);
                    }
                    setFY.add(dateFy);
                }
            } else {
                for (integer j = 1; j <=12; j++) {
                    if (j < 10) {
                        dateFy = i + '0' + j;
                    } else {
                        dateFy = i + '' + j;
                    }
                    setFY.add(dateFy);
                }
            }
        }
    }
    return setFY;
}