Hmm. I know more about the celts, but they certainly tracked lunar cycles and were quite sophisticated. 17 was a sacred number in accordance with the cycles, and modern historians take most of the roman accounts of their barbaric nature with a grain of salt. As they were not a writing and documenting culture, it's difficult to know with certainty. While the weekday names were certainly by most accounts borrowed from the Romans, they had an incredibly sophisticated knowledge of astronomy. With the recent surveys of Stonehenge revealing even more complexity and solstice correlation, it was, with certainty, a far more sophisticated culture in those parts than often presumed.
So, while they may not have aligned with western units of measure, they absolutely tracked time, seasons, cycles, and annual events with accuracy.
Very probably yes.
The oldest known Celtic Calendar we have is the Coligny Tablet which was discovered in South East France. This dates to the 2nd c AD when the area was Romanised, however there are key differences which point to older pre-Roman origins for the time-keeping system.
The names of the months are different (translations from The Celtic Tradition by Caitlin Matthews.)
Month Period Meaning
Samonios Oct / Nov Seed-fall
Dumannios Nov / Dec The Darkest Depths
Riuros Dec / Jan Cold-time
Anagantios Jan / Feb Stay-home-time
Ogronios Feb / Mar Time of Ice
Cutios Mar / Apr Time of Winds
Giamonios Apr / May Shoots-show
Simivisionios May / Jun Time of Brightness
Equos Jun / Jul Horse-time
Elembiuos Jul / Aug Claim-time
Edrinios Aug / Sep Arbitration-time
Cantios Sep / Oct Song-time
The months begin on the full moon, whereas Roman months began around the new moon (the Kalends) and the Romans had their 'Ides' around the full moon.
There's also evidence to be found in Celtic languages. The Welsh word for week 'wythnos' literally means 'eight nights', indicating the periods of darkness (nights) were counted rather than the periods of light (days).
We don't have direct evidence but the circumstantial evidence would suggest that it's very likely that the Insular Celts had an organised time-keeping system just as their continental cousins most probably did.
http://en.wikipedia.org/wiki/Coligny_calendar
Celtic culture: a historical encyclopedia, edited by John T. Koch
http://www.roman-britain.org/celtic/coligny.htm
http://en.wikipedia.org/wiki/Roman_calendar#Months
NB - There's lots of 'reconstructed' Celtic Calendars around which take a bit of Colingy terminology, a sprinkling of Welsh Medieval tradition, a pinch of madey-up-stuff and mix it all together (or some other such combination) these are fine for those who wish to play make-believe druids but should not be treated as historically sound at all.