{namespace rf=DigiComp\FluidRenderFunctions\ViewHelpers}

<rf:registerRenderFunction as="renderDate">
    {subject -> f:format.date(format: 'd.m.Y')}
</rf:registerRenderFunction>
<f:form action="extendedTextField">
    <f:form.textfield value="{now}" renderFunction="renderDate" />
</f:form>
<f:form object="{post}" objectName="post" action="extendedTextfield">
    <f:form.textfield property="publishedAt" renderFunction="renderDate" />
</f:form>