My code may be enhanced.
During my first tests, I created the pathname in the main code.
It issued an error claiming that I had no rights to do that.
So I wrote the handler asking System Events to create a file.
In fact it was useless. What is needed is just to build the pathname out of the tell application "Numbers" block.
tell application "Numbers"
repeat
set theFiles to file of every document
--> {file "<myHD>:Users:<myAccount>:Desktop:truc.numbers", missing value, missing value}
if theFiles does not contain missing value then exit repeat
repeat with i from 1 to count theFiles
if item i of theFiles is missing value then
save document i in file (my createAfile())
delay 1 # to be sure to use a different name
exit repeat
end if
end repeat
end repeat
end tell
#=====
on createAfile()
set p2d to path to desktop as text
set newName to my horodateur(current date) & ".numbers"
return p2d & newName
end createAfile
#=====
on horodateur(une_date)
tell une_date to return (((its year) * 10000 + (its month) * 100 + (its day)) as text) & "_" & text 2 thru -1 of ((1000000 + (its hours) * 10000 + (its minutes) * 100 + (its seconds)) as text)
end horodateur
#=====