It is clear that You want to change the Crystal report header header dynamically . to do so , you have to take two
in your crystal report .and bind those dates to the corresponding TextObjects before you load the report.
CrystalReport1 sr = new CrystalReport1();
TextObject txtheader = (TextObject)sr.ReportDefinition.ReportObjects["txtheaderobject"];
txtheader.Text = "date";
TextObject txtheader1 = (TextObject)sr.ReportDefinition.ReportObjects["txtheaderobject1"];
txtheader1.Text = "date1";
crystalReportViewer1.ReportSource = sr;