BUG/MINOR: clock/stats: also use start_time not start_date in HTML info

For an unknown reason in the change of uptime calculation for the HTML
page didn't make it to commit 6093ba47c ("BUG/MINOR: clock: do not mix
wall-clock and monotonic time in uptime calculation"). Let's address it
as well otherwise the stats page will display an incorrect uptime.

No backport needed unless the patch above is backported.
1 file changed