Get Quarter Starting and Ending Dates for a given Date

To calculate the quarter, you can use the Time Period Library for .NET:

// ----------------------------------------------------------------------
public static void ShowQuarterInfo( YearMonth yearBaseMonth = YearMonth.January )
{
  ShowQuarterInfo( DateTime.Now, yearBaseMonth );
} // ShowQuarterInfo

// ----------------------------------------------------------------------
public static void ShowQuarterInfo( DateTime moment, YearMonth yearBaseMonth = YearMonth.January )
{
  // set start month to year
  TimeCalendar calendar = new TimeCalendar(
    new TimeCalendarConfig { YearBaseMonth = yearBaseMonth } );

  // working quarter
  Quarter quarter = new Quarter( moment );
  Console.WriteLine( "Quarter start: " + quarter.FirstDayStart );
  Console.WriteLine( "Quarter end: " + quarter.LastDayStart );

  // previous quarter
  Quarter previousQuarter = quarter.GetPreviousQuarter();
  Console.WriteLine( "Previous Quarter start: " + previousQuarter.FirstDayStart );
  Console.WriteLine( "Previous Quarter end: " + previousQuarter.LastDayStart );

  // next quarter
  Quarter nextQuarter = quarter.GetNextQuarter();
  Console.WriteLine( "Next Quarter start: " + nextQuarter.FirstDayStart );
  Console.WriteLine( "Next Quarter end: " + nextQuarter.LastDayStart );
} // ShowQuarterInfo

You will find more samples in the article, Time Period Library for .NET[^]. Cheers, Jani.