Error in the rowfilter

With this code :

DataView view = new DataView(data.syn_vAllActions);
            view.RowFilter =
                  "heure_rdv>='" + celldate1.ToString() + "' and heure_rdv<'" + celldate2.ToString() + "' and heure_rdv2<='" + celldate2.ToString() +"' and heure_rdv2>'" + celldate1.ToString() + "'" ;

I have this error message :

System.Data.EvaluateException was unhandled by user code
  Message="Cannot perform '>=' operation on System.DateTime and System.String."
  Source="System.Data"
  StackTrace:
       at System.Data.BinaryNode.SetTypeMismatchError(Int32 op, Type left, Type right)
       at System.Data.BinaryNode.BinaryCompare(Object vLeft, Object vRight, StorageType resultType, Int32 op)
       at System.Data.BinaryNode.EvalBinaryOp(Int32 op, ExpressionNode left, ExpressionNode right, DataRow row, DataRowVersion version, Int32[] recordNos)
       at System.Data.BinaryNode.Eval(DataRow row, DataRowVersion version)
       at System.Data.BinaryNode.Eval(ExpressionNode expr, DataRow row, DataRowVersion version, Int32[] recordNos)
       at System.Data.BinaryNode.EvalBinaryOp(Int32 op, ExpressionNode left, ExpressionNode right, DataRow row, DataRowVersion version, Int32[] recordNos)
       at System.Data.BinaryNode.Eval(DataRow row, DataRowVersion version)
       at System.Data.BinaryNode.Eval(ExpressionNode expr, DataRow row, DataRowVersion version, Int32[] recordNos)
       at System.Data.BinaryNode.EvalBinaryOp(Int32 op, ExpressionNode left, ExpressionNode right, DataRow row, DataRowVersion version, Int32[] recordNos)
       at System.Data.BinaryNode.Eval(DataRow row, DataRowVersion version)
       at System.Data.BinaryNode.Eval(ExpressionNode expr, DataRow row, DataRowVersion version, Int32[] recordNos)
       at System.Data.BinaryNode.EvalBinaryOp(Int32 op, ExpressionNode left, ExpressionNode right, DataRow row, DataRowVersion version, Int32[] recordNos)
       at System.Data.BinaryNode.Eval(DataRow row, DataRowVersion version)
       at System.Data.DataExpression.Invoke(DataRow row, DataRowVersion version)
       at System.Data.Index.AcceptRecord(Int32 record, IFilter filter)
       at System.Data.Index.InitRecords(IFilter filter)
       at System.Data.Index..ctor(DataTable table, Int32[] ndexDesc, IndexField[] indexFields, DataViewRowState recordStates, IFilter rowFilter)
       at System.Data.Index..ctor(DataTable table, IndexField[] indexFields, DataViewRowState recordStates, IFilter rowFilter)
       at System.Data.DataTable.GetIndex(IndexField[] indexDesc, DataViewRowState recordStates, IFilter rowFilter)
       at System.Data.DataTable.GetIndex(String sort, DataViewRowState recordStates, IFilter rowFilter)
       at System.Data.DataView.UpdateIndex(Boolean force, Boolean fireEvent)
       at System.Data.DataView.UpdateIndex(Boolean force)
       at System.Data.DataView.SetIndex2(String newSort, DataViewRowState newRowStates, DataExpression newRowFilter, Boolean fireEvent)
       at System.Data.DataView.SetIndex(String newSort, DataViewRowState newRowStates, DataExpression newRowFilter)
       at System.Data.DataView.set_RowFilter(String value)
       at PlanningDay.FillPlanningCell(String starttime, DataGridItemEventArgs e, Int32 colindex, Int32 cellinterval) in c:\Documents and Settings\sdd\My Documents\Visual Studio 2005\WebSites\e-Vision 2005\PlanningDay.ascx.cs:line 388
       at PlanningDay.DataGridDay_ItemDataBound(Object sender, DataGridItemEventArgs e) in c:\Documents and Settings\sdd\My Documents\Visual Studio 2005\WebSites\e-Vision 2005\PlanningDay.ascx.cs:line 358
       at System.Web.UI.WebControls.DataGrid.OnItemDataBound(DataGridItemEventArgs e)
       at System.Web.UI.WebControls.DataGrid.CreateItem(Int32 itemIndex, Int32 dataSourceIndex, ListItemType itemType, Boolean dataBind, Object dataItem, DataGridColumn[] columns, TableRowCollection rows, PagedDataSource pagedDataSource)
       at System.Web.UI.WebControls.DataGrid.CreateControlHierarchy(Boolean useDataSource)
       at System.Web.UI.WebControls.BaseDataList.OnDataBinding(EventArgs e)
       at System.Web.UI.WebControls.BaseDataList.DataBind()
       at PlanningDay.LoadControl() in c:\Documents and Settings\sdd\My Documents\Visual Studio 2005\WebSites\e-Vision 2005\PlanningDay.ascx.cs:line 707
       at Planning.Page_Load(Object sender, EventArgs e) in c:\Documents and Settings\sdd\My Documents\Visual Studio 2005\WebSites\e-Vision 2005\Planning.aspx.cs:line 54
       at System.Web.UI.Control.OnLoad(EventArgs e)
       at System.Web.UI.Control.LoadRecursive()
       at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

       What is wrong ? What should I do ?