Thanks for your question and interest in Forma 36. This is something that we have in our internal Forma 36 backlog, however I’m unable to give any estimations as to when the component will be available.
For now I’d recommend using a third-party datepicker. We provide design tokens and CSS helper packages as part of Forma 36 (https://github.com/contentful/forma-36/tree/master/packages) which could assist in styling a third-party datepicker.
Alternatively you could also use the native type attribute on an input component, although this comes with some browser support issues https://caniuse.com/#feat=input-datetime.
We will update the TextInput component to allow these attributes, once this has been updated you’d be able to use this component as an interim solution.