what is the purpose of date -d

I saw this in the man page, but the help doesnt clear things much for
me.Can any body tell me what it does

Re: what is the purpose of date -d

On 2007-10-05, parag_paul@hotmail.com wrote:
> I saw this in the man page, but the help doesnt clear things much for
> me.Can any body tell me what it does
>
Assuming that you are referring to the GNU version of date, "info date"
has more complete documentation.
Instead of the current date and time, you choose the date. You can
write it as "January 1, 1970" or "1970-1-1" "yesterday", "tomorrow", "3
days ago"; and the output can be formatted as in other date commands
with "+%Y%m%d" and so on.