class DateFilter : C1.Win.FlexGrid.IC1ColumnFilter

DateTime _min = DateTime.MinValue;
DateTime _max = DateTime.MaxValue;

/// <summary>
/// Gets or sets the minimum date required by the filter.
/// </summary>
public DateTime Minimum
{
  get { return _min; }
  set { _min = value; }
}
/// <summary>
/// Gets or sets the maximum date required by the filter.
/// </summary>
public DateTime Maximum
{
  get { return _max; }
  set { _max = value; }
}

#region ** IC1ColumnFilter Members

// filter is active if range doesn't cover all represetable dates.
public bool IsActive
{
  get { return _min != DateTime.MinValue || _max != DateTime.MaxValue; }
}

// reset filter.
public void Reset()
{
  _min = DateTime.MinValue;
  _max = DateTime.MaxValue;
}

// apply filter to a given date
public bool Apply(object value)
{
  var dt = (DateTime)value;
  return dt >= _min && dt <= _max;
}

// return editor control for this filter
IC1ColumnFilterEditor IC1ColumnFilter.GetEditor()
{
  return new DateFilterEditor();
}

#endregion 

public partial class DateFilterEditor :
   UserControl,
   C1.Win.FlexGrid.IC1ColumnFilterEditor

#region ** fields

DateFilter _filter;

#endregion

//-------------------------------------------------------------------------------
#region ** ctor

public DateFilterEditor()
{
  InitializeComponent();
}

#endregion

#region ** IC1ColumnFilterEditor

        public void Initialize(C1.Win.FlexGrid.C1FlexGridBase grid, int columnIndex, C1.Win.FlexGrid.IC1ColumnFilter filter)
        {
            _filter = (DateFilter)filter;
        }

        public void ApplyChanges()
        {
            // Begin ApplyChanges() method by resetting the filter states when a new filter is applied.
            _filter.Reset();

                // Set filter rules for last week filter
                if (_chkLastWeek.Checked)
                {
                    _filter.Maximum = DateTime.Today.AddDays(-7);
                    // This while loop will EXCLUDE Saturday from the result
                    while (_filter.Maximum.DayOfWeek != DayOfWeek.Saturday)
                    {
                        _filter.Maximum = _filter.Maximum.AddDays(1);
                    }
                    _filter.Minimum = DateTime.Today.AddDays(-7);
                    // This while loop will INCLUDE Monday from the result.
                    while (_filter.Minimum.DayOfWeek != DayOfWeek.Monday)
                    {
                        _filter.Minimum = _filter.Minimum.AddDays(-1);
                    }
                }

                // Set filter rules for last Fiscal Year. In this case, we want to say that the Fiscal Year starts April 1st and ends March 31st.
                if (_chkLastFiscalYear.Checked)
                {
                    _filter.Maximum = DateTime.Today.AddYears(-1);
                    _filter.Minimum = DateTime.Today.AddYears(-1);

                    // Set maximum filter to end at March 31st of current year.
                    while (_filter.Maximum.Month != 4 || _filter.Maximum.Day != 1)
                    {
                        _filter.Maximum = _filter.Maximum.AddDays(1);
                    }

                    // Set minimum filter to start at April 1st of last year.
                    while (_filter.Minimum.Month != 4 || _filter.Minimum.Day != 1)
                    {
                        _filter.Minimum = _filter.Minimum.AddDays(-1);
                    }
                }

                // Set the filter rules for this Fiscal Year. In this case, we want to say everything from the most recent April 1st up the present. Thus, we need two internal if statements to determine if April has already passed for this year.
                if(_chkThisFiscalYear.Checked)
                {
                    // Set the maximum to the current day + 1, since Maximum Bound for the filter is exclusive.
                    _filter.Maximum = DateTime.Today.AddDays(1);
                    // If today's month is January through March, then subtract a year from the minimum. For example, if today is January 2023, that means Fiscal Year started April 2022, so we have to use the AddYears(-1).Year when setting the DateTime for the minimum filter value.
                    if(DateTime.Today.Month < 4)
                    {
                        _filter.Minimum = new DateTime(DateTime.Today.AddYears(-1).Year, 4, 1);
                    }
                    // If today's month is April through December, then use the current year for "This Fiscal Year" starting point (minimum filter value) by simply using DateTime.Today.Year to set the year, as shown below.
                    if(DateTime.Today.Month >= 4)
                    {
                        _filter.Minimum = new DateTime(DateTime.Today.Year, 4, 1);
                    }
                }
            }
        }
        public bool KeepFormOpen
        {
            get { return false; }
        }

#endregion

c1FlexGrid1.Cols["Ship Date"].Filter = new DateFilter();