Which makes time seem longer?: The units of time (years, months, weeks, days, hours, minutes, seconds) or the number of units?

i.e.: I know someone that is counting down to Christmas. When it was 9 months away, that seemed long to me. But when I think of it as 273 days, somehow, that doesn't seem quite as long. That's why when I'm really excited about an upcoming event (such as my trip to Europe), I count down days.