How do I convert correctly timezones

Question

I am using the fasttime package for its fastPOSIXct function that can read character datetimes very efficiently. My problem is that it can only read character datet

Answer 1

Could you not just add the appropriate number of seconds to correct the offset from GMT?

# Original problem
fastPOSIXct("2010-03-15 12:37:17.223",tz="America/Montreal")
# [1] "2010-03-15 08:37:17 EDT"

# Add 4 hours worth of seconds to the data. This should be very quick.
fastPOSIXct("2010-03-15 12:37:17.223",tz="America/Montreal") + 14400
# [1] "2010-03-15 12:37:17 EDT"