Here is the WHOIS record for Lightning.Network domain:
Updated Date: 2019-02-02T01:41:17Z Creation Date: 2015-02-21T22:36:22Z Registry Expiry Date: 2020-02-21T22:36:22Z
Here is video presentation of Lightning Network from march 2015:
By February 2015 Lightning Network was already long under way.
I also remember the first draft of their Whitepaper and it was from end of 2014. Unfortunately it is not possible to find it currently.
Or is it? Can you find it? Prize awaits.
OK, most probably I am misremembering and off by few months, the publicly available whitepapers are from 2015 or later...
If nobody finds the older version until tomorrow, I will just split the reward between 2 people who convinced me of the truth.