According to what I read Skynet become aware was pushed back from the later date due to the out come of T2.
a result of temporal interference by Sarah Connor, her son John, Miles Dyson, and the T-800 destroying Cyberdyne headquarters and all backups of the research in 1995, the date for Judgment Day is moved back to here. Skynet is destined to go online a few days earlier on April 19, 2011, at 20:11.