[smokeping-users] What's mean unknown_sec ?

[Calvin Park]

Hello SmokePing Users ~

Below info. is RRD schema.

name            ( ex : uptime, loss ... )
type              ( ex : GAUGE, COUNTER... )
min. heart beat	 ( ex : 300, 600 )
min	
max	
last_DS	
value	
unknown_sec   <--- this

Could you explain to me what's used for 'unknow_sec' field ?

Thank you . Have a good weekends ~