Stats clock drift

The base code of stats data collection logic assumes the clock of host OS to be steady.  However, in a system with severe clock drift problem and the system is configured with cron job to readjust its clock not long before the scheduled stats data collection time, admin-ui's stats table can end up collecting the stats data twoice per data collection cycle.  So for example, when the admin-ui is scheduled to run stats data collection once a day, the stats table can pick up two entries with split seconds apart per day.   This PR gives admin-ui some ability to cope with clock fluctuation in the host OS.