It's visit's first action time.
To find such visits, you can create a segment with particular referrer and then go through the visitor log to find exact time&date.
But I'd like to create statistics with all referrers and date/time of each, is it possible?
Thanks in advance
Yes, you can use the Live API see: http://developer.piwik.org/api-reference/reporting-api#Live and https://piwik.org/docs/real-time/#real-time-analytics-api