Use bash built-in printf rather than external date command

[Megver83]

Bash built-ins tend to be faster than external commands and preferred for shell scripts. date can be easily replaced with printf time format, like this example:

$ time date +%Y-%m-%d
2022-05-28
real    0m0,001s
user    0m0,001s
sys     0m0,000s
$ time printf "%(%Y-%m-%d)T"
2022-05-28
real    0m0,000s
user    0m0,000s
sys     0m0,000s

It kinda keeps the date format. Plus, it can be used to directly set variables with the -v option.
Just type printf --help for more info :D

[pedroetb]

I will take a look into this for next release, thanks!