There is no hard and fast rule to format an on-screen countdown clock. As with all general formatting advice, your goal is to express your idea clearly and simply, without taking the reader out of the scene. Any way you can accomplish that is acceptable.
Your first instinct may be to mark each appearance of the clock as an [INSERT](https://screenwriting.io/what-is-an-insert/) or [SUPER](https://screenwriting.io/what-does-super-mean/). This may be appropriate for the clock’s introduction, but would most likely be distracting after the first appearance. Instead, consider using [slugs](https://screenwriting.io/what-is-a-slug/) to mark time.
INT. PINE CREST GYM – LATER
INSERT: 8 seconds remain on the scoreboard. We’re tied at 77.
Claudia is at the foul line.
She bounces the ball twice, lines up, and shoots.
It circles the rim and falls. No good.
CLAUDIA
Crud.
She takes off after her own rebound.
Bodies crash, but she parts the sea.
7 SECONDS LEFT
She gets a hand on the ball, but the St. Thomas PG snatches it away and takes off towards the other end of the court.
5 SECONDS
Claudia is in hot pursuit.
4 SECONDS
She catches up to the PG, and with a quick swipe, Claudia knocks the ball free before the PG knows what hit her.
2.5 SECONDS
On a dime, Claudia changes directions, sprinting back towards her basket, the ball in her possession.
1 SECOND
Just over half-court, she heaves it towards the basket --
BUZZZZZ!
-- and it sinks as time expires!
It also may be possible to include the clock without calling attention to it so obviously, both in its introduction and in its recurrences.
EXT. THE LIBRARY – DAY
Jade helps direct traffic as patrons evacuate.
JADE
Single file. Orderly, please!
As she waves people through, she notices something out of the corner of her eye, hidden under a bush.
She approaches, and finds
A BACKPACK.
She has a bad feeling about this.
Jade reaches down and opens it, holding her breath as she does.
And as she suspected, she sees
THE BOMB.
Fifteen seconds until detonation.
JADE
(to herself)
Great.
Determining there’s no point to even try to clear the area, she rolls up her sleeves and dives in.
JADE (CONT’D)
Eeny, meeny, miny... Ahh, screw it.
With ten seconds until detonation, she snips the red wire.
The clock starts moving at double speed.
JADE (CONT’D)
That’s not good.
Panicking, she reaches in and rips all the wires out.
The clock continues to tick quickly.
Thinking fast, she jumps on the backpack and holds tight.
Keep in mind that your inclusion of the specifics of the countdown is strictly for flavor. Ultimately, the director and editor will determine how to handle it.
* [Writing fight scenes](http://johnaugust.com/2011/writing-fight-scenes “Writing fight scenes”)
* [Avoiding cliches](http://johnaugust.com/2003/avoiding-cliches “Avoiding cliches”)
* [Does the editor even read the script?](http://johnaugust.com/2006/does-the-editor-even-read-the-script “Does the editor even read the script?”)