Just an ammendment to my previous post: my calculations were using east coast Australian time (GMT+10) whereas the Unix timestamp is in GMT. Therefore Active Directoy's "accountexpires" integer value does start from 1-Jan-1601 00:00:00 GMT and the number of seconds between this date and 1-Jan-1970 00:00:00 GMT is 11644524000.
The increments are still definately in 100 nanoseconds though!