First, going to give you a different viewpoint than fixing the date variable:
Cloning is not a good backup. UUIDs get changed, which may interfere with activated software. Also, since things change the files cannot be FC'd or hashed to ensure backup integrity.
The best backup is a full file & folder copy of a completely shut-down (not save-stated) guest folder, containing all the files therein, as well as any disk files that may reside outside the folder. The backup can be FC file-compared, or hashes can be taken and compared for off-site backup where FC would not be practical. If the disk file(s) are inside the folder right next to the guest's .vbox file, then that backup can be restored to any capable Virtualbox host. If a disk file is outside the guest folder, then the .vbox file contains an absolute path to the disk file, and that path must be re-created on the new host (or the .vbox file needs to be manually edited).
As for dates in a file name, here is a snippet of Windows batch file code (I got off Stack Exchange I think) that renames a file to a time-stamped name:
rename myfile.txt "myfile_%date:~10,4%_%date:~4,2%_%date:~7,2%_%time:~0,2%_%time:~3,2%_%time:~6,5%.txt"
I think the "%date:~X,Y%" means, starting at the Xth character of the output of 'date', take Y characters". This because usual date/time outputs contain characters that aren't allowed in Windows filenames. Linux most likely has an equivalent function.