convert 12 time stamp to 24 hour

str(testtimedataset)

With my real data set, revealed that after conversion, the time series
data consists of 'chr' which I guess means character type data,
reporting the following error:

Error in plot.window(...) : need finite 'xlim' values
In addition: Warning messages:
1: In xy.coords(x, y, xlabel, ylabel, log) : NAs introduced by coercion
2: In min(x) : no non-missing arguments to min; returning Inf
3: In max(x) : no non-missing arguments to max; returning -Inf


Does this mean I must convert the 24-hour time series values to
POSIX*t type objects; if so how please?

Anyway, by

plot(testdata~testtimedataset$V1)

you will get rid of error message but depending on testtimedataset mode you
can get different one.

Confirmed, thanks.