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Thats because the single quotes prevent variable substitution. Use double quotes instead:
$ x=$(date +%H:%M)
$ echo $x
16:46
$ TZ=UTC date -d "${x} EST" '+%Y%m%d %H:%M'
20121206 21:46

You might want to know what you did wrong to arrive at something different as expected:

Matches in "sed" are always "greedy". That means of different possible matches always the longest possible...