SQL Server uses 8 bytes to store a datetime value—the first 4 for the date and the second 4 for the time. SQL Server can interpret both sets of 4 bytes as integers. For the date portion, the value SQL Server stores is the number of days before or after a base date of January 1, 1900. Because of this storage protocol, SQL Server assumed the date of January 1, 1900, when you didn't supply the date, SQL Server internally stored a value of 0. A negative number represents a date earlier than January 1, 1900.
SQL Server stores the second integer for the time as the number of clock ticks after midnight. A second contains 300 ticks, so a tick equals 3.3 milliseconds (ms). You can see the values for days and clock ticks by converting a datetime value to a binary(8) value and using the substring function to extract each set of 4 bytes.