public static class DateExtensions
{
    public static string ToText(this DateTime date)
    {
        var hour = (DateField)date.Hour;
        bool zy = false, zm = false, zd = false;
        if ((hour & DateField.Year) > 0)
            zy = true;
        if ((hour & DateField.Month) > 0)
            zm = true;
        if ((hour & DateField.Day) > 0)
            zd = true;

        if (zd & zm)
            return $"год {date.Year} ";

        if (zd)
            return $"год {date.Year} месяц {date.Month}";

        if (zy)
            return $"год не указан";

        return date.ToShortDateString();
    }

    public static string ToText(this DateTime? date) => date.HasValue ? ToText(date.Value) : "";

    public static DateTime Build(int year, int month, int day)
    {
        DateField mask = 0;
        if (year == 0)
        {
            year = 1;
            mask = mask | DateField.Year;
        }

        if (month == 0)
        {
            month = 1;
            mask = mask | DateField.Month;
        }

        if (day == 0)
        {
            day = 1;
            mask = mask | DateField.Day;
        }

        return new DateTime(year, month, day, mask, 0, 0);
    }

    [Flags]
    enum DateField
    {
        Year = 1,
        Month = 2,
        Day = 4
    }
}