[Repeating for convenient reference a comment in an earlier thread.] R 3.4.2 was in code freeze when High Sierra as released, and so there was no opportunity to make and test workarounds for undocumented changes to the latter. R will be unable to find the system timezone on High Sierra (confirmed with the CRAN 3.4.2 release): > Sys.timezone() [1] NA > .leap.seconds [1] "1972-07-01 GMT" "1973-01-01 GMT" "1974-01-01 GMT" "1975-01-01 GMT" [5] "1976-01-01 GMT" "1977-01-01 GMT" "1978-01-01 GMT" "1979-01-01 GMT" [9] "1980-01-01 GMT" "1981-07-01 GMT" "1982-07-01 GMT" "1983-07-01 GMT" [13] "1985-07-01 GMT" "1988-01-01 GMT" "1990-01-01 GMT" "1991-01-01 GMT" [17] "1992-07-01 GMT" "1993-07-01 GMT" "1994-07-01 GMT" "1996-01-01 GMT" [21] "1997-07-01 GMT" "1999-01-01 GMT" "2006-01-01 GMT" "2009-01-01 GMT" [25] "2012-07-01 GMT" "2015-07-01 GMT" "2017-01-01 GMT" Warning message: In as.POSIXlt.POSIXct(x, tz) : unknown timezone 'default/Europe/London' Perhaps the best workaround is to use R-patched from http://r.research.att.com/ . However, in almost all cases a workaround is to set TZ, either outside the session (not so easy for R.app) or at the beginning of the session by e.g. Sys.setenv(TZ = "Europe/London") (where the correct value will be obvious from running the code above).
Brian D. Ripley, ripley at stats.ox.ac.uk Emeritus Professor of Applied Statistics, University of Oxford