[R] difftime error

[dpender]

R community,

I am trying to create an array of the time differences between datapoints
for a very large set.  For some reason for 4 of the values the difference
has been calculated as NA.

Looking at the individual points two of them are "1981-03-29 01:40:00" and
"1981-03-29 02:00:00"

This is the exact same format as the other points that return values of 20
mins as expected.

In such a large dataset (600,000+ entries) I don't know why only 4 would be
doing this.

Apologies for such a vague question but can anyone shed any light on what
may be wrong?

Thanks, 

Doug
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