i'm interested by the plugin flag_thumb_as_new (http://www.zenphoto.org/2010/02/flag_thumb_as_new/)
i don't understand the differences between the 2 options "date" and "mtime".

could you explain a little ?

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.