iirc SharePoint should handle the date conversion if you use the same approach as you do with new Property Mappings.
Copy/paste a variable in the template, something like:
var modifiedDate = $getItemValue(ctx, "ModifiedOWSDATE");
modifiedDate.overrideValueRenderer($contentLineText);
And use that variable instead:
<p>_#= modifiedDate =#_</p>
If creating a new custom Value Renderer etc is not an option for formatting the date (The default pattern used by Search.ClientControls isLongDatePattern
I think). One could always create a Date variable, and do as fit:
Example:
<p>_#= new Date(modifiedDate["inputValue"]).toLocaleString() =#_</p>