How can I change the format of a date from the command line?

Question

What\'s the quickest way to convert a date in one format, say

2008-06-01

to a date in another format, say

Answer 1

date -d ... 

doesn't seem to cut it, as I get a usage error:

usage: date [-jnu] [-d dst] [-r seconds] [-t west] [-v[+|-]val[ymwdHMS]] ... 
            [-f fmt date | [[[mm]dd]HH]MM[[cc]yy][.ss]] [+format]

I'm running GNU bash, version 3.2.17(1)-release (i386-apple-darwin9.0), and as far as the man goes, date -d is just for

-d dst  Set the kernel's value for daylight saving time.  If dst is non-
         zero, future calls to gettimeofday(2) will return a non-zero for
         tz_dsttime.