Manti Te'o Girlfriend Hoax -- Witness Talks ... New Timeline Emerges

Manti Te'o and his non-existent girlfriend Lennay Kekua went public with their relationship in January 2012 ... and continued to date until she died in September ... TMZ has learned.

TMZ spoke with "Jan" ... a Notre Dame superfan who struck up an online friendship with "Lennay" through Twitter back in 2011.

According to Jan, Lennay had always maintained that she was friendly with Manti ... but didn't reveal that she was his "girlfriend" until January.

The timeline begs one really big question ... in 9 months, two college students at top universities (Lennay maintained she went to Stanford) didn't have one video chat session??