How do you get how many days passed from the previous Tuesday (or any day name) until DateTime.Now o-CodePudding

Not sure how to go about this. I can find todays date.

string now = System.DateTime.Now.DayOfWeek.ToString();
Console.WriteLine("Today is: "   now);

then I have a string that could be any day of the week but no date for it. So lets say all I have is:

string day = "Thursday";

How do I subtract that day (example being Thursday) from now (Monday or todays date 8/15/2022)? What I'm after is the actual date. In this case it would be 8/11/2022 Even getting the number of days from any given day till now would be helpful.

CodePudding user response：

You can maintain an offset map of System.DayOfWeek that allows you to offset DateTime.Now based on the current DayOfWeek.

var now = DateTime.Now;
var dayOfWeek = now.DayOfWeek;
    
var offset = dayOfWeek switch
{
    DayOfWeek.Monday => -6,
    DayOfWeek.Sunday => -5,
    DayOfWeek.Saturday => -4,
    DayOfWeek.Friday => -3,
    DayOfWeek.Thursday => -2,
    DayOfWeek.Wednesday => -1,
    DayOfWeek.Tuesday => -7, // or 0, depending on your needs;
    _ => 0
};

var lastTuesday = now.AddDays(offset);

Console.WriteLine(lastTuesday.Date); // Outputs 8/9/2022 12:00:00 AM

CodePudding user response：

You can simply use enum calculations:

    int DaysSince(DayOfWeek dow)
    {
        var today = System.DateTime.Now.DayOfWeek;
        return (7   today - dow) % 7;
    }

    var daysSince = DaysSince(DateOfWeek.Tuesday);

CodePudding user response：

Note that I have based this answer on the actual question, that says you want a date, rather than the title, which says you want a count of days. If you follow the method below, you can increment a variable in the loop to get a count of days.

Here's one option:

private static DateTime GetDateOfLastDayOfWeek(string dayName)
{
    if (!Enum.TryParse(dayName, false, out DayOfWeek dayOfWeek))
    {
        throw new ArgumentException("Not a valid day of week", nameof(dayName));
    }

    var date = DateTime.Today;

    do
    {
        date = date.AddDays(-1);
    } while (date.DayOfWeek != dayOfWeek);

    return date;
}

Note that, if today is the specified day, it will go back to the previous instance. If you wanted to match today's date, you could use a while loop instead of a do:

while (date.DayOfWeek != dayOfWeek)
{
    date = date.AddDays(-1);
}