Share forms have each field as an external ftl.
Those are in config/alfresco/site-webscripts/org/alfresco/components/form/controls/*
The one you're interested in is config/alfresco/site-webscripts/org/alfresco/components/form/controls/date.ftl
And potentially config/alfresco/site-webscripts/org/alfresco/components/form/controls/daterange.ftl
If you want to change all the date fields across alfresco, just overriding this component in web-extensions would be fine.
If you want to keep both, you'll need to create a distinct component and choose to use your custom components by redefining the form definition in share-config-custom.xml