WPF Datepicker MVVM Light passing selected date to viewmodel

I'm new to this and I'm at my wits end so I'm reaching out for help.

I'm trying to pass the selected dates to a property that I can then use in a sql query, but the dates past are the default and the the newly selected dates. Below is my xaml, viewmodel and sqlquery class. Any help would be greatly appreciated.

<Window
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        DataContext="{Binding Main, Source={StaticResource Locator}}"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" mc:Ignorable="d" x:Class="NSP_Commission.MainWindow"
        Title="NSP Commission Tracker" Height="350" Width="525"><Grid><Grid.RowDefinitions><RowDefinition Height="40"/><RowDefinition Height="*"/></Grid.RowDefinitions><Label Margin="5,5,6.103,9" HorizontalAlignment="Left" Width="110" Content="Select Date Range:"/><DatePicker x:Name="dpBeginDate" HorizontalAlignment="Left" Margin="120,5,0,9" Width="120" SelectedDate="{Binding BeginDate, Mode=TwoWay}"/><DatePicker x:Name="dpEndDate" HorizontalAlignment="Left" Margin="270,5,0,9" Width="120" SelectedDate="{Binding EndDate, Mode=TwoWay}"/><TextBox Text="To" HorizontalAlignment="Left" Margin="242,6,0,10" Width="28" BorderBrush="{x:Null}"/><Button x:Name="ImportButton" Content="Import" HorizontalAlignment="Right" Margin="0,8,5,10" Width="75" d:LayoutOverrides="Height" Command="{Binding ImportData}"/></Grid></Window>

namespace NSP_Commission.ViewModel
{

    public class MainViewModel : ViewModelBase
    {
        public MainViewModel()
        {
            if (IsInDesignMode)
            {
                // Code runs in Blend --> create design time data.
            }
            else
            {
                ImportData = new RelayCommand(() => ImportDataExecute(), () => true);   
            }   
        }

        public ICommand ImportData { get; private set; }
        private void ImportDataExecute()
        {
            Invoice_Hdr ih = new Invoice_Hdr();
            ih.LoadInvoice_Hdr();
            Invoice_Dtl id = new Invoice_Dtl();
            id.LoadInvoice_Dtl();

            MessageBox.Show("Import Complete!");
        }

        private DateTime _beginDate = DateTime.Today;
        public DateTime BeginDate
        {
            get { return _beginDate; }
            set
            {
                _beginDate = value;
                RaisePropertyChanged("BeginDate");
            }
        }

        private DateTime _endDate = DateTime.Today;
        public DateTime EndDate
        {
            get { return _endDate; }
            set
            {
                _endDate = value;
                RaisePropertyChanged("EndDate");
            }
        }
    }   
}

namespace NSP_Commission.CustomCode.SqlQuerys
{
    class Invoice_Hdr : MainViewModel
    {
        public string LoadInvoice_Hdr()
        {
            string myqueryString;
            DateTime myBeginDate;
            DateTime myEndDate;

            //Assign Report Parameters Based on Logged In User
            //SelectedDates sd = new SelectedDates();
            
            myBeginDate = BeginDate;
            myEndDate = EndDate;

            myqueryString = "blah blah " + BeginDate + ";

            return myqueryString;
        }
    }
}