Question: when does hl2 take place?
Answer: HL1 takes place anywhere between 2000 and 2009. HL2 takes place 20 years later (as stated by official Valve sources, the writers, and shown by the characters' aging), so anywhere inbetween 2020-2029. Around 2000-2009 2015-2029 because Alyx was only a baby during the black-mesa incident.