With shell builtins:

• zsh:

   $ zmodload zsh/datetime $ TZ=UTC0 strftime %c 1282368345 Sat 21 Aug 2010 05:25:45 UTC 

• ksh93:

   $ TZ=UTC0 printf '%(%c)T\n' '#1282368345' Sat Aug 21 05:25:45 2010 

• bash:

   $ TZ=UTC0 printf '%(%c)T\n' 1282368345 Sat 21 Aug 2010 05:25:45 UTC 

In all those, replace %c with the strftime() format you want.

With shell builtins:

• zsh (4.1.1 (2003) and above) :

   $ zmodload zsh/datetime $ TZ=UTC0 strftime %c 1282368345 Sat 21 Aug 2010 05:25:45 UTC 

• ksh93 (ksh93i (2000) and above):

   $ TZ=UTC0 printf '%(%c)T\n' '#1282368345' Sat Aug 21 05:25:45 2010 

• bash (4.2 (2010) and above):

   $ TZ=UTC0 printf '%(%c)T\n' 1282368345 Sat 21 Aug 2010 05:25:45 UTC 

In all those, replace %c with the strftime() format you want.