[DefaultValue(null)]
public DateTime? Start { get; set; }

<DefaultValue(Nothing)>
Public Property Start As Date?

<dx:ASPxGantt ID="Gantt" runat="server" ... /> 
    ...
    <WorkTimeRules> 
        <dx:DailyRule>
            <Recurrence Start="2/22/2019" End="2/25/2019" />
            <WorkTimeRanges>
                <dx:WorkTimeRange Start="08:00" End="10:00" />
                <dx:WorkTimeRange Start="15:00" End="18:00" />
            </WorkTimeRanges>
        </dx:DailyRule>
        ...
    </WorkTimeRules>
</dx:ASPxGantt>

DailyRule rule1 = new DailyRule();
rule1.WorkTimeRanges.AddRange( new List<WorkTimeRange>
{
    new WorkTimeRange(){ Start=new TimeSpan(08,00,00), End=new TimeSpan(10,00,00)},
    new WorkTimeRange(){ Start=new TimeSpan(15,00,00), End=new TimeSpan(18,00,00)}
});
rule1.Recurrence.Start = new DateTime(2019, 02, 22);
rule1.Recurrence.End = new DateTime(2019, 02, 25);

ASPxGantt1.WorkTimeRules.Add(rule1);