以下是一个按月计数并返回包含13个值的字符串的示例代码：

import datetime

def count_months(start_date, end_date):
    start_month = start_date.month
    end_month = end_date.month
    months = []

    for year in range(start_date.year, end_date.year + 1):
        if year == start_date.year:
            start = start_month
        else:
            start = 1

        if year == end_date.year:
            end = end_month
        else:
            end = 12

        for month in range(start, end + 1):
            months.append(datetime.date(year, month, 1).strftime('%b'))

    return ' '.join(months)

# 示例用法
start_date = datetime.datetime(2022, 1, 1)
end_date = datetime.datetime(2022, 12, 31)
result = count_months(start_date, end_date)
print(result)

运行上述代码，将输出以下结果：

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

这是一个示例，你可以根据具体需求进行修改和适配。