To expand on points above...
Welsh has two forms of its numbering system, a traditional vigesimal (20-based) one, and the simplified ten-based system that kids are first taught in schools (where 31 is literally described as "three-tens-one"). The traditional system is now used mainly in dates (e.g. the suffix in "17th" of the month) and times, e. g. 25 past the hour would be 'pum munud ar hugain wedi' - "five minutes on twenty past".
The numbers from eleven upward have different names in the old vigesimal system. Only eleven ('unarddeg') and twelve are commonly used for naming the hours, though twelve doesn't get that much of a look in, as those hours are usually described as 'hanner dydd' and 'hanner nos' for midday and midnight respectively. For the minutes, vigesimal 20 and 25 are commonly used (for the multiples of 5 that aren't covered by "quarter" or "half past") but many speakers will use the modern "one-on-ten" system for describing other (or any) number of minutes, as they may not be fully comfortable with the vigessimal system any more.
This means that Welsh is in an odd position of using traditional names for times, but the names for hours from 13 upward aren't really in common use anywhere, so it'd be strange or unfamiliar for many people to hear or use them.
As an aside, the way 24-hour times are read out in English is a radically different system to the 12-hour clock: there are no minutes to or past the hour, it's just a case of reading the number of hours, thenn the number of minutes. (Nobody says it's nine minutes to eighteen o' clock, they'd say seventeen fifty-three.) This works great for brevity and clear communication, and is why the US knows the 24-hour clock as "Army time". I assume it was heavily used in the British army as well prior to it's more public adoption for railway timetables. But this usage wouldn't have existed in Welsh, as the British army operates in English, for obvious reasons.
Given that this second (24 hour) clock-reading system hadn't been used already in Welsh, inventing/adopting one for the purpose of railway platform announcements would have felt like a very odd and unnatural choice.
Another tidbit on Welsh times: we actually have three Welsh postscripts to indicate the period of day for a 12-hour clock: y.b. (y bore), y.p. (y pnawn) and y.h. (yr hwyr), for morning, afternoon, and late/evening respectively. Many people use the latin a.m. and p.m. by default instead, which I feel is a shame given that we actually have Welsh versions (whereas English doesn't).
And now to drift into money matters for a moment:
The above term "chwe ugain" described by
@sefyllian was used for the paper note ('papur chweugain') and later for the 50p piece ('darn chweugain'), though usage of these terms is decreasing over time, as the pre-decimal origin is foreign to people of even my age. I only know of it because my father used it on occasion, and explained its origin.
Other terms for pre-decimalisation coins include 'swllt' for shilling, 'dimai' for halfpenny, and 'coron' for a crown (a literal translation). Some like 'grôt' and 'fflorin' were direct equivalents to the English terms, and there'd be plenty of other local terms that I've not come across. Only swllt and dimai would be familiar to the youth of today, because they're used in a few popular nursery rhymes.
As a final tidbit, the term used for money/coins differs in North and South Wales dialects. In the south, and in official writing, the term use is 'arian' (silver), whereas up north it's described as 'pres' (brass). I find this an interesting parallel with Northern English dialects that use "brass" to describe money.