Nice question. I enjoyed it.

The "flying duck" code layout made it easy to check that the parentheses matched up, so could see easily that there was no syntax error unless it was caused by those strange literals. So I had to find out what those strange literals were before going any further. First I hit on Date, Time, and Timestamp Literals but the nasty typo on that page ("-type" for "literal-type") confused me so I looked further and found Date, Time, and Timestamp Escape Sequences which gave me what I needed (and showed clearly that my confusion with the earlier page was due to a typo) so now I knew there wasn't a syntax error.

So then I could follow a process much like what some others have mentioned. Since a lot of things were added, all integer multiples of one second and none of them zero, the miliseconds in the result had to be the same as in the starting point (non-zero) and the resulting datetime had to be greater than the starting point - so only one of the options fitted the bill. No need to do any arithmetic - but it made me think!