Accurately converting from character->POSIXct->character with sub millisecond datetimes

Question

I have a character datetime column in a file. I load the file (into a data.table) and do things that require the column to be converted to POSIXct.

Answer 1

Two things:

1) @statquant is right (and the otherwise known experts @Joshua Ulrich and @Dirk Eddelbuettel are wrong), and @Aaron in his comment, but that will not be important for the main question here:

POSIXlt by design is definitely more accurate in storing times than POSIXct: As its seconds are always in [0, 60), it has a granularity of about 6e-15, i.e., 6 femtoseconds which would be dozens of million times less granular than POSIXct.

However, this is not very relevant here (and for current R): Almost all operations, notably numeric ones, use the Ops group method (yes, not known to beginners, but well documented), just look at Ops.POSIXt which indeed trashes the extra precision by first coercing to POSIXct. In addition, the format()/print() ing uses 6 decimals after the "." at most, and hence also does not distinguish between the internally higher precision of POSIXlt and the "only" 100 nanosecond granularity of POSIXct.
(For the above reason, both Dirk and Joshua were lead to their wrong assertion: For all simple practical uses, the precision of *lt and *ct is made the same).

2) I do tend to agree that we (R Core) should improve the format()ing and hence print()ing of such fractions of seconds POSIXt objects (still after the bug fix mentioned by @Aaron above).
But then I may be wrong, and "we" have got it right, by some definition of "right" ;-)