With shell builtins:
zsh:$ zmodload zsh/datetime $ TZ=UTC0 strftime %c 1282368345 Sat 21 Aug 2010 05:25:45 UTCksh93:$ TZ=UTC0 printf '%(%c)T\n' '#1282368345' Sat Aug 21 05:25:45 2010bash:$ 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.