Calculating date field with just a year double field [QGIS]

I have a dataset in QGIS that I originally created in ArcMap. It has two fields for dates, let’s call them ‘date_from’ and ‘date_to’. Both of these fields contain only a year i.e. ‘2012’ and nothing else, this was originally sufficient for my needs. They are of the double type.

Upon moving over to QGIS I see that dates need to be in the format ‘yyyy-mm-dd’. How would I best go about creating new fields to add the full dates using what I already have? Note that the first of Jan and last of Dec would be sufficient for the new ‘from’ and ‘to’ fields.