I believe `date` comes from the data in the EXIF/IPTC field of the same name (which can be changed from the admin back-end after uploading). `mtime` (which stands for "modified time") is the time something was last modified.
So using `date` would flag an image as new if the image's date is recent and `mtime` would flag it as new if it was recently modified.
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So using `date` would flag an image as new if the image's date is recent and `mtime` would flag it as new if it was recently modified.