Hi Eric,
Thanks for using Syncfusion products.
We can convert the date object to double value in AddBreak method by using ToOADate() method of datetime object.please refer below code snippet.
[C#]
DateTime start = new DateTime(2011, 5, 27);
this.ChartWebControl1.PrimaryXAxis.AddBreak(start.ToOADate(), start.AddDays(4).ToOADate());
We have prepared a simple sample and the same can be downloaded from the following link.
http://www.syncfusion.com/uploads/redirect.aspx?&team=support&file=Chart1038386969.zipPlease let us know if any concerns.
Regards,
Siva Kumar D