标签：string,C#,所在,获取,time,年月日 From： https://www.cnblogs.com/IBPM/p/17104355.html
/// <summary>
/// 获取该时间所在月份的最后一天的string(年月日)
/// </summary>
/// <param name="time"></param>
/// <returns></returns>
public static string get_last_day_at_parameters_time(this DateTime time) => (time.AddMonths(1).ToString("yyyy-MM") + "-01").to_datetime().AddDays(-1).to_stringxymd();