Date in DateField with year resolution switches to BC after entering 0

We are using Vaadin 7.5.7. Problem still persists in 7.7.6

Reproduction steps:

1. Create a DateField
2. Set its resolution as Resolution.YEAR
3. Enter 0 into this DateField and switch focus

After that any selected or inputed year is interpreted as year BC.

Example UI:

@Theme("mytheme")
public class MyUI extends UI {

    @Override
    protected void init(VaadinRequest vaadinRequest) {
        final VerticalLayout layout = new VerticalLayout();

        // Main part - create date field with year resolution
        DateField dateField = new DateField();
        dateField.setCaption("Field with year resolution");
        dateField.setResolution(Resolution.YEAR);

        // Add control to show the value currently in dateField
        Button button = new Button("Show value in field");
        button.addClickListener(event -> {
            SimpleDateFormat formatWithEra = new SimpleDateFormat("yyyy.MM.dd G 'at' HH:mm:ss z");
            Label label = new Label(formatWithEra.format(dateField.getValue()));
            layout.addComponent(label);
        });

        layout.addComponents(dateField, button);
        layout.setMargin(true);
        layout.setSpacing(true);
        setContent(layout);
    }
}

Reproduce the issue using a Vaadin DateField configured with Resolution.YEAR, entering 0, and changing focus. Start by tracing DateField's year-resolution handling and compare the resulting value with the SimpleDateFormat output shown in the example. Done means entering 0 no longer causes later selected or entered years to be interpreted as BC.