To set date format we will need dconf-editor tool, standart features doesn't support custom format.
$ sudo apt install dconf-tools
and start tool:
Find path in tree: com -> canonical -> indicator -> datetime and change params:
custom-time-format: %Y-%m-%d %H-%M time-format: custom
That's it. Now we have custom formated date in applet like 2016-07-25 15:31.