Here is a picture of two people on a date:
The language we use to represent the picture depends upon discourse choices. The picture may be represented as a series of completed events or, in other words, a story. In this case the primary tense would mainly be used:
Last week, Jun and Mae went on their first date. First, they went to a fancy Chinese restaurant in Yokohama for dinner. After dinner, they left the restaurant and walked arm-in-arm through the colourful streets of China Town. It was still quite early so they went to a coffee shop and talked for hours.
On the other hand, it may also be represented as a series of interconnected events that unfold through time. For example, if the photographer were describing the picture:
Here’s a photo of my friends on their first date. They look cute together, don’t they! I think they were walking through China Town, in Yokohama here. I think so. Yeah, they had just finished dinner and were walking through China Town to a coffee shop where they were going to have coffee.
Of course, it may also be possible to combine the two, in which case the primary tense unfolds the main events while the secondary tense adds more detail:
Last week, Jun and Mae went on their first date. They had met at a lecture in college three months before and had been flirting ever since. First, they went for dinner to a fancy Chinese restaurant in Yokohama – it had even appeared on TV. After dinner, they left the restaurant and walked arm-in-arm through the colourful streets of China Town. Lots of other couples were walking there too. It was still quite early so they went to a nice coffee shop that they had noticed before and talked there for hours.
The choice of tense, then, is not so much a function of time but one of discourse – it is a realization of how the speaker of writer wishes to represent the event.