Only in an interactive bash shell (i.e. the command line), an exclamation mark followed by an alphanumeric character does history substitution.
For example
echo "!1"prints line 1 from the command...
The sed does not find anything.
Your mistake is the 'ticks' around $dt, but the shell only expands $dt within "quotes".
Further, a /x/,/x/ range is always one line, so you can simply address it...