What does this phrasal verb mean: 'to clock up'? Is it an idiom?

[Nasreddin]

Test No. incompl/inter-78 "Tick Tock", question 8

I've clocked ......... about an additional 20 hours this week in overtime pay.

(a) on
(b) for
(c) up
(d) with

Test No. incompl/inter-78 "Tick Tock", answer 8

I've clocked up about an additional 20 hours this week in overtime pay.

Correct answer: (c) up

Your answer was: n/a
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What does it mean clocked up?

[Torsten]

To clock up means to accumulate - to reach a large number (of hours, kilometres etc.)

TOEIC listening, talks: Weather forecast on radio

[Gheon]

Hey guys, is it me or there's a typo in this sentence?
"I've clocked up about an additional 20 hours this week in overtime pay."
The only possible explanation for an here is that an additional 20 hours means some period of time. Please, comment on the case.
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I find your Lack of Faith disturbing!
The Force is out there!

[Esl_Expert]

Hi Gheon

Interesting things often happen to singular and plural in grammar when we talk about amounts. In the phrase '20 hours', '20' is a number, but '20 hours' is no longer just a number; instead it is an amount of time. In essence, the use of 'an additional' signifies 'one more of this particular amount of time'.

You might also be interested in reading this thread from another ESL forum:
article before plural noun
;-)
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"No man can climb out beyond the limitations of his own character." ~ John Morely

[Gheon]

Gramercy, Amy!
For the explanation & for the informative link as well.
Though I must confess of the sin of rereading all info several times before I got it ;)
_________________
I find your Lack of Faith disturbing!
The Force is out there!

[Dongocan]

lock up sth:to reach a particular amount or number

[Kati Svaby]

More example:

On the trip we clocked up over 1800 miles.
He has clocked up more than 25 years on the committee.

[Kati Svaby]

Correction: More examples: