[R] mean for vector with NA

[bad2101 at columbia.edu]

I am trying to find the mean for the elements in the vector

Incubation=as.POSIXlt(OnsetTime)-as.POSIXlt(MealTime)

where

OnsetTime=c(NA,"1940-04-19 00:30","1940-04-19 00:30","1940-04-19  
00:30",NA,"1940-04-18 22:30","1940-04-18 22:30","1940-04-19  
02:00","1940-04-19 01:00","1940-04-18 23:00",NA,NA,NA,"1940-04-19  
02:00",NA,"1940-04-19 10:30","1940-04-19 00:30","1940-04-18  
22:15",NA,"1940-04-18 22:00","1940-04-19 01:00","1940-04-18  
23:00",NA,"1940-04-18 21:45",NA,"1940-04-18 21:45","1940-04-19  
01:00",NA,"1940-04-18 23:00",NA,"1940-04-18 21:00","1940-04-19  
01:00","1940-04-19 01:00",NA,NA,"1940-04-18 21:15",NA,"1940-04-18  
23:30","1940-04-19 01:00","1940-04-18 21:30",NA,"1940-04-19  
02:30","1940-04-19 02:00","1940-04-18 21:30",NA,NA,"1940-04-19  
00:30","1940-04-19 01:00","1940-04-18 22:30",NA,NA,"1940-04-18  
15:00",NA,"1940-04-19 00:00","1940-04-18 23:00",NA,"1940-04-18  
22:30","1940-04-19 01:00","1940-04-19 02:30","1940-04-18  
23:30",NA,NA,NA,NA,"1940-04-19 01:00","1940-04-19  
00:30",NA,NA,NA,"1940-04-19 00:30","1940-04-19 01:00","1940-04-19  
00:00",NA,"1940-04-19 02:15","1940-04-18 23:00")
MealTime=c(NA,"1940-04-18 20:00","1940-04-18 18:30","1940-04-18  
18:30",NA,"1940-04-18 19:30","1940-04-18 19:30","1940-04-18  
19:30","1940-04-18 22:00","1940-04-18 19:00",NA,NA,NA,"1940-04-18  
19:30",NA,NA,NA,NA,NA,NA,"1940-04-18 22:00",NA,NA,NA,NA,NA,"1940-04-18  
22:00",NA,NA,"1940-04-18 22:00",NA,"1940-04-18 22:00","1940-04-18  
22:00",NA,NA,NA,NA,NA,"1940-04-18 22:00",NA,NA,NA,NA,NA,"1940-04-18  
22:00",NA,NA,"1940-04-18 19:00",NA,NA,NA,"1940-04-18  
11:00",NA,NA,NA,NA,NA,"1940-04-18 22:00","1940-04-18  
19:30","1940-04-18 19:30",NA,NA,NA,NA,"1940-04-18  
22:00",NA,"1940-04-18 19:30","1940-04-18 19:30",NA,NA,"1940-04-18  
19:30","1940-04-18 19:30","1940-04-18 22:00",NA,NA)

I have tried

mean(Incubation)

and

mean(as.numeric(Incubation))

but I think that, since there are so many NA values in Incubation, R  
gives a mean value of NA.  Is there any way of either extracting all  
numeric values (i.e. all non-NA values) from the Incubation vector, or  
finding the mean value of only the numeric values in the Incubation  
vector?

Brian