question...

Would they be referring to the passage of time through the film? The characters meet up from week to week so maybe a lot is left to the viewer to almost second guess or make presumptions about the characters' emotional developments during that time not shown?

I'd expect they'd be referring to the non-linear, "flashback" structure of the film since the very beginning of the film is actually its ending.

Time is important everywhere in the film, from Albert Godby looking at his watch as the express passes through to Alec Harvey looking at his watch when they only have a few minutes left at the end.

Time represented as a killer in this film. These two meet at the wrong time in their lives and that is what kills their romance. The whole movie is about how little time these two have to spend together. They are always being rushed and pressured to catch their train back home. At one point one of them states how short life is. They just never have enough time together.