Updates to default timezone setting in datetime functionality

[cmccarthy1]

This change closes #63

[ralmgren1]

Did this ever get merged into the main branch? The code I pull still has the old version, with the timezone set to GMT.
I can see that this might fail the test in testthat/test-basic.R line 73, which explicitly expects timezone GMT, though on my installation it seems to pass all the tests. I would suggest changing line 72 of test-basic.R from

rdatetime <- as.POSIXct('2006-07-21 09:13:39', tz='GMT')

to

rdatetime <- as.POSIXct('2006-07-21 09:13:39', tz='UTC')
attr(rdatetime,'tzone') <- NULL

and then the test expect_equal(datetime, rdatetime) in current line 73, new line 74, should succeed.