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.)
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