This code logs a task based on the current disposition field, it also schedules a task based on a follow up required check box and a follow up date field. I am creating the tasks fine, but dont seem to be able to set the ActivityDate. Can some one tell me what I am doing wrong?

{!requireScript("/soap/ajax/13.0/connection.js")}

try
{
var account = new sforce.SObject("Account");
var task = new sforce.SObject("Task");
var followUp = new sforce.SObject("Task");

account.id = "{!Account.Id}";
account.IsDirty__c = true;

// set task 'assigned to' field to the current user
task.OwnerId = "{!Account.OwnerId}";
task.Subject = "{!Account.Current_Disposition__c}";
task.WhatId = "{!Account.Id}";
task.Description = "{!Account.Notes__c}";

// set status to closed
task.Priority = "Normal";
task.Status = "Completed";

// if follow up required, then create the follow up task
if ({!Account.Follow_Up_Required__c})
{
followUp.OwnerId = "{!Account.OwnerId}";
followUp.Subject = "Follow Up Call";
followUp.WhatId = "{!Account.Id}";
followUp.Description = "{!Account.Notes__c}";

// set status to closed
followUp.Priority = "Normal";
followUp.Status = "Not Started";
followUp.IsReminderSet = true;
followUp.ActivityDate = Date.valueOf({!Account.Follow_Up_Date__c});
}

var result = sforce.connection.update([account]);
var resultT = sforce.connection.create([task]);
var resultF = resultT;

if (followUp != null)
{
resultF = sforce.connection.create([followUp]);
}

if (result[0].getBoolean("success") && resultT[0].getBoolean("success") && resultF[0].getBoolean("success"))
{
account.IsDirty__c = false;
account.Follow_Up_Required__c = false;
account.Follow_Up_Date__c = null;
account.Current_Disposition__c = null;
account.Notes__c = null;

result = sforce.connection.update([account]);

if (result[0].getBoolean("success"))
{
window.location.reload();
}
}
else
{
alert("Error!");
}
}// End try block
catch(err)
{
alert("Error creating task: " + err.toString());
}