The short answer is yes (unless you have a camera that can output PNG files (which, I believe, is very few)).
The long answer is (in general) sort of. As mentioned in the comments, file conversion is, technically, a form of alteration and so, for the vast majority of photos, having a .PNG extension means that they has been altered. This does not mean that the content of the photograph has been altered (at least, in a human sense), and not having a .PNG file extension does not mean that the file hasn't been edited. Your PNG files may still have some EXIF data that includes Capture date and time, camera model etc. that will allow you to determine and get some way to proving the actual time the photo was taken.
The conversion to PNG could come from where and how you are saving the files after the photos have been taken. For instance, some cloud services may automatically convert image files to PNG (maybe for compression, etc).