I have a webservice with a method which is called via a xmlhttprequest object in my javascript. The method accepts a datetime parameter which is subsequently converted to a string and run against the database to perform a calculation.

I get the value from m_txtDateAdd and send off the xmlHttprequest

<asp:textbox id=m_txtDateAdd tabIndex=4 runat="server" Width="96px" Text="<%# Today %>">
</asp:textbox>

which has a validator attacted to it

<asp:CustomValidator id="m_DateAddValidator" runat="server" ErrorMessage="Please Enter a Valid Date" ControlToValidate="m_txtDateAdd">&#x25CF;</asp:CustomValidator>

My webmethod looks something like this

[WebMethod]
public decimal GetTotalCost(DateTime transactionDate)
{
    String sqlDateString = transactionDate.Year+"/"+transactionDate.Month+"/"+transactionDate.Day;

I use sqlDateString as part of the commandtext i send off to the database. Its a legacy application and its inline sql so I don't have the freedom to set up a stored procedure and create and assign parameters in my code behind. This works 90% of the time. The webservice is called on the onchange event of m_txtDateAdd. Every now and again the response i get from the server is

System.ArgumentException: Cannot convert 25/06/2009 to System.DateTime. System.ArgumentException: Cannot convert 25/06/2009 to System.DateTime.

Parameter name: type ---> System.FormatException: String was not recognized as a valid DateTime.

   at System.DateTimeParse.Parse(String s, DateTimeFormatInfo dtfi, DateTimeStyles styles)
   at System.DateTime.Parse(String s, IFormatProvider provider)
   at System.Convert.ToDateTime(String value, IFormatProvider provider)
   at System.String.System.IConvertible.ToDateTime(IFormatProvider provider)
   at System.Convert.ChangeType(Object value, Type conversionType, IFormatProvider provider)
   at System.Web.Services.Protocols.ScalarFormatter.FromString(String value, Type type)
   --- End of inner exception stack trace ---
   at System.Web.Services.Protocols.ScalarFormatter.FromString(String value, Type type)
   at System.Web.Services.Protocols.ValueCollectionParameterReader.Read(NameValueCollection collection)
   at System.Web.Services.Protocols.HtmlFormParameterReader.Read(HttpRequest request)
   at System.Web.Services.Protocols.HttpServerProtocol.ReadParameters()
   at System.Web.Services.Protocols.WebServiceHandler.CoreProcessRequest()