Kabuto still remembers the first night in Orochimaru's service when he looked at the clock and it was after 9 PM.
He remembers that it was 11:17 PM exactly, and he remembers that he had been performing a titration in the lab when he was so tired that he couldn't even see straight when he had stopped for no apparent reason to check the time.
It hadn't been intentional, some symbolic conscious choice to move from one phase of his life to another. It had just happened, and the epoch was complete.
He remembers that he didn't sleep that night.