(Credit also to Rob Hardy)

When using strptime function in R, by default, R recognizes strings (and then converts them into time format) based on your current locale setting of your OS.

Hence, to resolve the NA issue, you have to change the locale options of R, or otherwise change that of your OS. According to this StackOverflow post, you can do that by:

Sys.setlocale("LC_ALL","English")

Or, you can also change “LC_TIME” instead of chaging “LC_ALL”. The latter alters all locale-related settings.

Sys.setlocale("LC_TIME", "en")          # Solaris 7: details are OS-dependent
Sys.setlocale("LC_TIME", "en_US.utf8")  # Modern Linux etc
Sys.setlocale("LC_TIME", "en_US")       # Mac OS
Sys.setlocale("LC_TIME", "English")     # Windows