The expression is mostly self-explanatory and does not need to make date-to-string-substring-to-date conversions to achieve the same effect.
Internally - datetime is just 8-byte FLOAT number as stored by SQL Server with right section of the decimal point representing number of dates since 01-Jan-1900 (hence 0 is 01 Jan 1900 12:00AM) and fractional section is the time (0.5 is 12:00 PM, 0.25 is 4:00 AM, 0.75 is 4:00PM etc).