My 'simple' answer is also the complicated one - you use whatever works - whatever looks right. In some cases - such as the countdown - it has more to do with 'other' aspects of the writing than focusing on the time.
feigning interest in Mr Böringer's lecture, Hannah looked over at the time on the computer. Five minutes past three. Another ten minutes before the bell, if only she could keep her eyes open.
The clock on the wall read 11:25, blinked off and then on again, then read 11:24. Tom did a double take, looking around the room. How could no one else notice?
The timer was counting down. Fourteen. The screwdriver was slipping, slick in her hands. Thirteen. The plate was off, she didn't see where it fell, only the mass of wiring attached to the detonator. Twelve. In the movies it's always the red or green wire, these ones were all white. Eleven. ...
I generally put it in numbers (5:44 AM), but if it's in dialog, I'll write it out in words (five forty-four AM).
There's the older generation's way of saying it---quarter of six, half past five, and so on. Remember that digital clocks are a recent innovation and for a long time a clock was a dial with "hands" that moved.
I'd write it out based on the feeling you are trying to capture with the use of time. Time can be used to communicate boredom or urgency or simply to place the time of day.
Personally, I feel like using the "old school" method, half past six, quarter to ten, etc... is more informal and less exact, so I would not use it if I was trying to lend a sense of urgency.
Writing it out like a digital clock feel more precise, uses fewer characters, and therefore to me can feel more urgent, though obviously the words around the time will control that more so than how you write the time.
Just write what feels right, I think the words around the time are more important than how you write the time itself.
quote: The timer was counting down. Fourteen. The screwdriver was slipping, slick in her hands. Thirteen. The plate was off, she didn't see where it fell, only the mass of wiring attached to the detonator. Twelve. In the movies it's always the red or green wire, these ones were all white. Eleven. ...
I like this form of countdown it works well, It keeps the action flowing and at the same time allows the reader to experience the countdown.
Well, it depends. If the story involved a minute-crucial countdown then, the full time needs to be posted. Otherwise, a fifteen minute or even half hour estimation of the time should be fine. You can also tie it into the story without having to specifically mention the time. (Frequently interrupting the story to tell the reader what time it is might get annoying)
If the character is walking down the street, the "four o'clock bus" could be pulling up to unload passengers. If its out in the country, the character could hear the whistle blast of the five o'clock train. In a house? ten long chimes punctuated the quiet of the night.