To piggyback on what @Vemonus said above:
According to the Half-Life Universe Timeline -
"Between 2015 and 2029
Day 1 of Gordon's journey:
◾Under two decades after the Black Mesa Incident, Gordon Freeman is awakened by the G-Man after a long period of stasis, he is placed on a train entering the City 17 Train station.
◾Barney prevents Gordon from entering a prisoner train and reintroduces both himself and Dr. Kleiner to Gordon."
All the way to the end of Day 1 with:
"◾Combine forces led by Breen raid Black Mesa East while Alyx, Dog, and Gordon are in the nearby scrapyard. Gordon escapes the raid and enters the nearby town of Ravenholm. Eli is captured and taken to Nova Prospekt. Dog and Alyx also escape the raid.
◾Gordon meets Grigori, Ravenholm's priest. Gordon uses Grigori's traps to explore the town."
There was a bit of a "switcheroo" from Valve because initially they made the story to assume Gordan Freeman started work on May 15th somewhere between 2000-2009. But in the Half-Life Episodes they allude to the time that Gordan once spent in Black Mesa...
"◾This day was originally Gordon Freeman's first day of work at Black Mesa, but it was RetConn'ed in Half-Life 2 and its episodes, where it is implied that Gordon was already working for some time at Black Mesa before the incident: Barney tells Gordon he owes him a beer and reminds him who he is when first meeting in the City 17 Train station, and Alyx tells in the City 17 Underground that Gordon and Barney would compete in air ducts to get into Kleiner's office whenever he got himself locked out, among others."
I hope this helps out!