Date conversion query

I've come into possession of hundreds of ascii data files where the date and time are separate columns like so:

date     time
1-Jan-08  23:05 

I need to convert this to a usable R Date object, subtract 8 hours (timezone conversion from UTC to Pacific) and then turn it into unix time. I need to do this since the data are collected every evening (from 5pm through 2am the following morning). So if I were to use regular date/time format it would confound day开发者_JS百科s (day1 spans two days when in fact it was just one evening of data collection). I'd like to consider each day's events separately.

Using unixtime will allow me to calculate time differences in events that occur each day (I will probably retain a date field in addition to the unix time). Can someone suggest an efficient way to do this?

Here is some data to use (this is in UTC)

dummy=data.frame(date="1-Jan-08",time="23:05")  

---

Paste them together (which works vectorised) and then parse, e.g.

  datetime <- paste(dummy$date, dummy$time)
  parsed <- strptime(datetime, "%d-%b-%y %H:%M")

which you can also assign as columns in the data frame.

Edit: strptime() has an optional tz="" argument you can use.