Compute the DateTime of an upcoming weekday

Very simple sample to include or exclude current date, you specify the date and the day the week you are interested in.

public static class DateTimeExtensions
{
    /// <summary>
    /// Gets the next date.
    /// </summary>
    /// <param name="date">The date to inspected.</param>
    /// <param name="dayOfWeek">The day of week you want to get.</param>
    /// <param name="exclDate">if set to <c>true</c> the current date will be excluded and include next occurrence.</param>
    /// <returns></returns>
    public static DateTime GetNextDate(this DateTime date, DayOfWeek dayOfWeek, bool exclDate = true)
    {
        //note: first we need to check if the date wants to move back by date - Today, + diff might move it forward or backwards to Today
        //eg: date - Today = 0 - 1 = -1, so have to move it forward
        var diff = dayOfWeek - date.DayOfWeek;
        var ddiff = date.Date.Subtract(DateTime.Today).Days + diff;

        //note: ddiff < 0 : date calculates to past, so move forward, even if the date is really old, it will just move 7 days from date passed in
        //note: ddiff >= (exclDate ? 6 : 7) && diff < 0 : date is into the future, so calculated future weekday, based on date
        if (ddiff < 0 || ddiff >= (exclDate ? 6 : 7) && diff < 0)
            diff += 7; 

        //note: now we can get safe values between 0 - 6, especially if past dates is being used
        diff = diff % 7;

        //note: if diff is 0 and we are excluding the date passed, we will add 7 days, eg: 1 week
        diff += diff == 0 & exclDate ? 7 : 0;

        return date.AddDays(diff);
    }
}

some test cases

[TestMethod]
    public void TestNextDate()
    {
        var date = new DateTime(2013, 7, 15);
        var start = date;
        //testing same month - forwardOnly
        Assert.AreEqual(start = start.AddDays(1), date.GetNextDate(DayOfWeek.Tuesday)); //16
        Assert.AreEqual(start = start.AddDays(1), date.GetNextDate(DayOfWeek.Wednesday)); //17
        Assert.AreEqual(start = start.AddDays(1), date.GetNextDate(DayOfWeek.Thursday)); //18
        Assert.AreEqual(start = start.AddDays(1), date.GetNextDate(DayOfWeek.Friday)); //19
        Assert.AreEqual(start = start.AddDays(1), date.GetNextDate(DayOfWeek.Saturday)); //20
        Assert.AreEqual(start = start.AddDays(1), date.GetNextDate(DayOfWeek.Sunday)); //21
        Assert.AreEqual(start.AddDays(1), date.GetNextDate(DayOfWeek.Monday)); //22

        //testing same month - include date
        Assert.AreEqual(start = date, date.GetNextDate(DayOfWeek.Monday, false)); //15
        Assert.AreEqual(start = start.AddDays(1), date.GetNextDate(DayOfWeek.Tuesday, false)); //16
        Assert.AreEqual(start.AddDays(1), date.GetNextDate(DayOfWeek.Wednesday, false)); //17

        //testing month change - forwardOnly
        date = new DateTime(2013, 7, 29);
        start = date;
        Assert.AreEqual(start = start.AddDays(1), date.GetNextDate(DayOfWeek.Tuesday)); //30
        Assert.AreEqual(start = start.AddDays(1), date.GetNextDate(DayOfWeek.Wednesday)); //31
        Assert.AreEqual(start = start.AddDays(1), date.GetNextDate(DayOfWeek.Thursday)); //2013/09/01-month increased
        Assert.AreEqual(start.AddDays(1), date.GetNextDate(DayOfWeek.Friday)); //02

        //testing year change
        date = new DateTime(2013, 12, 30);
        start = date;
        Assert.AreEqual(start = start.AddDays(1), date.GetNextDate(DayOfWeek.Tuesday)); //31
        Assert.AreEqual(start = start.AddDays(1), date.GetNextDate(DayOfWeek.Wednesday)); //2014/01/01 - year increased
        Assert.AreEqual(start = start.AddDays(1), date.GetNextDate(DayOfWeek.Thursday)); //02
    }

I want to get next day from this time include today also I want it to be at 12:00 AM

  public static DateTime GetNextWeekday(DateTime start, DayOfWeek day , bool includetoday = false) {
        if (includetoday && start.DayOfWeek == day) return start.Date;
        int daysToAdd = ((int)day - (int)start.DayOfWeek + 7) % 7;
        return start.Date.AddDays(daysToAdd);
    }