First off, you need to use $calc to calculate the difference in the time.
alias test {
$iif($chan,.msg $chan,echo -a) $duration($calc($duration(19:00:00) - $duration($time)),3)
}
This will give an incorrect response if the time is after 7 pm, but other than that should give you what you're asking for.
Read the help file under $duration and $calc for more details as to how they work.