What was the time it took Gordon to get from the beginning of the story (anomalous materials facility of Black Mesa) to the Nihilanth? I believe he arrived in the morning but I don't recall walking around outside during night. Was it just another day at the office?
I believe the game takes place over a span of two days.
When Gordon goes outside past the rocket pit in chapter 8 (On a Rail), it is dark out:
In the next chapter Gordon is captured by the assassins. I would guess he was out for multiple hours, which is why it's light outside when he escapes:
Gordon then goes on to work through the facility some more, to the scientists, and then to Xen, all of which probably doesn't take more than a day.