You're so reliable <3

Here you go op, updated to what Loki said.
Code:
on *:text:!startuptime:#: {
  msg # Starting uptime.
  set -e %uptime. [ $+ [ # ] ] $ctime
}

on *:text:!uptime:#: { msg # $duration($calc($ctime - %uptime. [ $+ [ # ] ] )) }

on *:text:!stopuptime:#: { 
  msg # Stopping uptime.
  unset %uptime. [ $+ [ # ] ] 
}

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