Calculate the start-date and name of a quarter from a given date

I think this solution would work pretty well. It takes up more line, bus is very verbose! `

private DateTime GetFirstDayOfYearlyQuarter(DateTime value)
{
    switch (value.Month)
    {
        case 1:
        case 2:
        case 3:
            return new DateTime(value.Year, 1, 1);
        case 4:
        case 5:
        case 6:
            return new DateTime(value.Year, 4, 1);
        case 7:
        case 8:
        case 9:
            return new DateTime(value.Year, 7, 1);
        case 10:
        case 11:
        case 12:
            return new DateTime(value.Year, 10, 1);
        default:
            throw new Exception(@"¯\_(ツ)_/¯");
    }
}

`

P.S. This only finds first day of quarter, but you can easily extend this to find number of quarter, etc.

DateTime date;
int quarterNumber = (date.Month-1)/3+1;
DateTime firstDayOfQuarter = new DateTime(date.Year, (quarterNumber-1)*3+1,1);
DateTime lastDayOfQuarter = firstDayOfQuarter.AddMonths(3).AddDays(-1);

A simpler two liner with live demo here + press F8 to run .

var date = DateTime.Now; //Give you own DateTime
int offset = 2, monthsInQtr = 3;

var quarter = (date.Month + offset) / monthsInQtr; //To find which quarter 
var totalMonths = quarter * monthsInQtr;

var startDateInQtr = new DateTime(date.Year, totalMonths - offset, 1); //start date in quarter 

If you are looking at last day of the quarter use DateTime.DaysInMonth

var endDateInQtr = new DateTime(date.Year, totalMonths, DateTime.DaysInMonth(date.Year, totalMonths));

    public static class DateTimeExtensions
{
    public static DateTime StartOfQuarter(this DateTime _date)
    {
        var quarter = decimal.ToInt16(Math.Ceiling(_date.Month / 3.0m));
        return new DateTime(_date.Year, quarter, 1);
    }
}

use

var quarterStart = DateTime.Today.StartOfQuarter();