
is.na(strptime("19810329012000",format="%Y%m%d%H%M%S"))
[1] TRUE

The problem was to do with daylight saving time.  I need to specify a time zone as this time doesn't exist in my operating system's current time zone.  I still think this is odd behaviour though!  When you look at the missing object it doesn't look missing at all.  

Caroline

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