Some time conversions in R

Form a "POSIXlt" object from a string :

tt = strptime("20100101","%Y%m%d", tz="GMT")

Compute the seconds since 1970 ((Internal storage format of the "POSIXlt" class)

as.double(tt)

Compute the days since 1970 (Internal storage format of the "Date class)

as.double(as.Date(tt))

or

unclass(as.Date(tt)) 

Convert an xts object into seconds since 1970

xx = xts(111, tt)
as.double(time(xx))

Convert an xts object into days since 1970

floor(as.double(time(xx))/60/60/24)

Construct a time object from different numeric components

ISOdatetime(year, month, day, hour, min, sec, tz = "")

Extract from a POSIXlt object the Year, month, day  etc information in numeric format :

class(a)
[1] "POSIXlt" "POSIXt"

a
[1] "2009-07-06 20:00:28"

c(a$year+1900, a$mon+1, a$mday, a$hour, a$min, a$sec)
[1] 2009    7    6   20    0   28