Hi,

Let's build a little project to get you started:

create new IP project (with C# behind) using Visual Studio.
Drop 2 date picker controls on your page (you now have 2 fields in your
schema: field1 and field2).
For the purpose of this exercise, open you mayschema.xsd file and go to the
XML view.
remove the "nillable=true" lines to give:
<xsd:element name="field1" type="xsd:date"/>
<xsd:element name="field2" type="xsd:date"/>

Next, at the bottom of your FormCode.cs code page, copy the 2 functions
given to you in the blog:
private static DateTime Iso8601ToDate( string iso8601Date )
{
    if( iso8601Date == null ) throw new ArgumentNullException( "iso8601Date" );
    return DateTime.ParseExact( iso8601Date, "yyyy-MM-dd", null );
}
private static string DateToIso8601( DateTime dateTime )
{return dateTime.ToString( "yyyy-MM-dd" );}

Then, go to your IP form:
Right-click on the first Date Picker (field1), click on Data Validation,
select OnAfterChange from the event drop-down and click Edit.
Now insert the following code:
[InfoPathEventHandler(MatchPath="/my:myFields/my:field1",
EventType=InfoPathEventType.OnAfterChange)]
public void field1_OnAfterChange(DataDOMEvent e)
{
    if (e.IsUndoRedo){return;}
    IXMLDOMNode dateNode = thisXDocument.DOM.selectSingleNode( "//my:field2" );
    try
    {
        DateTime dt = Iso8601ToDate( e.Source.text );
        dt = dt.AddYears( 1 );
        dateNode.text = DateToIso8601( dt );
    }
    catch( FormatException ) {}
}

run your form, pick 1 date in Date Picker 1, Date Picker 2 fills up with the
same date + 1 year...

Voila!

Hope that it helps you.

Franck Dauché