Change exif date of photo with gthumb

To modify the Exif, Exchangeable image file format, of an image file, you can use jhead.

From a terminal, install it by running the command:

 sudo apt-get install jhead

jhead is used to display and manipulate data contained in the Exif header of JPEG images from digital cameras.

Synopsis

 jhead [ options ] [ file... ]

General Metadata Options

-te file --- Transplant Exif header from a JPEG (with Exif header) in file into the image that is manipulated. 
-dc --- Delete comment field from the JPEG header. 
-de --- Delete the Exif header entirely. 
-di --- Delete the IPTC section, if present. 
-dx --- Delete the XMP section, if present. 
-du --- Delete sections of jpeg that are not Exif, not comment, and otherwise not contributing to the image either.
-purejpg --- Delete all JPEG sections that aren't necessary for rendering the image. 
-mkexif ---  Creates minimal exif header. Exif header contains date/time, and empty thumbnail fields only. Date/time set to file time by default. the exif header this way. 
-ce --- Edit the JPEG header comment field.
-cs file --- Save comment section to a file 
-ci file ---  Replace comment with text from file 
-cl string ---  Replace comment with specified string from command line file 

Date / Time

 -ft --- Sets the file's system time stamp to what is stored in the Exif header.
 -dsft --- Sets the Exif timestamp to the file's timestamp. Requires an Exif header to pre-exist. Use -mkexif option to create one if needed.
 -n[format_string] ---  This option causes files to be renamed and/ or mmoved using the date information from the Exif header "DateTimeOriginal" field. 
If the format_string is omitted, the file will be renamed to MMDD-HHMMSS. 
If a format_string is provided, it will be passed to the strftime function as the format string. In addition, if the format string contains '%f', this will substitute the original name of the file (minus extension). '%i' will substitute a sequence number. Leading zeros can be specified like with printf - i.e. '%04i' pads the number to 4 digits using leading zeros.
If the name includes '/', this is interpreted as a new path for the file. If the new path does not exist, the path will be created.
If the target name already exists, the name will be appended with "a", "b", "c", etc, unless the name ends with a letter, in which case it will be appended with "0", "1", "2", etc.
This feature is especially useful if more than one digital camera was used to take pictures of an event. By renaming them to a scheme according to date, they will automatically appear in order of taking in most directory listings and image browsers. Alternatively, if your image browser supports listing by file time, you can use the -ft option to set the file time to the time the photo was taken.

Some of the more useful arguments for strftime are:

%H Hour in 24-hour format (00 - 23)
%j Day of year as decimal number (001 - 366)
%m Month as decimal number (01 - 12)
%M Minute as decimal number (00 - 59)
%S Second as decimal number (00 - 59)
%w Weekday as decimal number (0 - 6; Sunday is 0)
%y Year without century, as decimal number (00 - 99)
%Y Year with century, as decimal number

More info: https://linux.die.net/man/1/jhead

It seems you can use following:

1. select a photo/photos
2. click "tool" icon
3. click "change date"
4. mark "Comment date" and put required date into field "The following date"

It works for me in gThumb 2.13.0