[Konctantin]

public static DateTime EndOfYear(this DateTime dateTime)
{
    return new DateTime(dateTime.Year, 12, 31, 23, 59, 59);
}

public static DateTime StartOfYear(this DateTime dateTime)
{
    return new DateTime(dateTime.Year, 1, 1);
}

public static DateTime EndOfMonth(this DateTime dateTime)
{
    return new DateTime(dateTime.Year, dateTime.Month, 31, 23, 59, 59);
}

public static DateTime StartOfMonth(this DateTime dateTime)
{
    return new DateTime(dateTime.Year, dateTime.Month, 1);
}

public static DateTime EndOfDay(this DateTime dateTime)
{
    return new DateTime(dateTime.Year, dateTime.Month, dateTime.Day, 23, 59, 59);
}

public static DateTime StartOfDay(this DateTime dateTime)
{
    return new DateTime(dateTime.Year, dateTime.Month, dateTime.Day);
}

public static DateTime StartOfWeek(this DateTime dayInWeek)
{
    var dt = dayInWeek.Date;
    while (dt.DayOfWeek != DayOfWeek.Monday)
        dt = dt.AddDays(-1);
    return new DateTime(dt.Year, dt.Month, dt.Day);
}

public static DateTime EndOfWeek(this DateTime dayInWeek)
{
    var dt = dayInWeek.Date;
    while (dt.DayOfWeek != DayOfWeek.Sunday)
        dt = dt.AddDays(1);
    return new DateTime(dt.Year, dt.Month, dt.Day, 23, 59, 59);
}