Hello PMac,

By saying that you would like to disable a Button control whenever any of the XamDateTimeInputs have an invalid input, I presume you mean that the value of the input is not a valid DateTime instance.
If this is the case, since the XamDateTimeInput does not introduce a property that indicates whether the value is a valid DateTime object or not, an approach I can suggest you is to manually perform the validation operation in code-behind by handling the ValueChanged event of both Inputs with a shared handler.

XAML:

<ig:XamDateTimeInput x:Name="inputA" ValueChanged="input_ValueChanged" />
<ig:XamDateTimeInput x:Name="inputB" ValueChanged="input_ValueChanged" /> 

Code-behind:
    private void input_ValueChanged(object sender, EventArgs e)
{
            if (IsValidDateTime(inputA.Text) && IsValidDateTime(inputB.Text))
                button.IsEnabled = true;
            else
                button.IsEnabled = false;
}

private bool IsValidDateTime(string text)
{
            DateTime temp;
            return DateTime.TryParse(text, out temp);
}

This way whenever the value of one of the controls has been changed, the ValueChanged event will fire and the Button will get enabled or disabled respectively based on whether both controls have a valid DateTime value or not.

I have attached a sample application, where the approach from above has been used.

If you require any further assistance on this matter, please do not hesitate to ask. 

Hello PMac,

The InvalidValueBehavior exists in the WPF 14.1 Product version.

For more detailed information on the property, you can take a look at Using XamDateTimeInput and InvalidValueBehavior Property and make sure you are using all the required assemblies for the control.

• InfragisticsWPF4.v14.1.dll
• InfragisticsWPF4.Controls.Editors.XamCalendar.v14.1.dll
• InfragisticsWPF4.Controls.Editors.XamDateTimeInput.v14.1.dll
• InfragisticsWPF4.Controls.Editors.XamMaskedInput.v14.1.dll
  
  
  If you are still experiencing any issues on the matter, please let me know.

Hello PMac,

The red warning message you have referred to is caused by the inability of the ValueConverter to convert the current value by returning null.
In order to prevent this from happening, you can set the property you are binding to of a nullable type. This way the ValueConverter will accept the value whenever it is null as well and you should not be getting the message.
 
public DateTime? FirstValue { get; set; }
<ig:XamDateTimeInput x:Name="inputA"  Value="{Binding FirstValue}" />

I have modified and attached my previous sample by binding the first input to a nullable DateTime property and the second input to a normal DateTime property. This way you should be able to see the difference.
For more detailed information, you can take a look at the following thread, where a similar issue has been discussed.

If you require any further assistance on the matter, please let me know.