Yes, because without quotes that's getting treated as a mathematical expression, specifically as two numbers being subtracted from 2015.

2015 - 07 - 20 = 1988

The calculated value of 1988 is then implicitly cast to datetime, the way that work is the number is treated as days since 1900. 1988 days after the first of January 1900 is the 12th of June 1905

Therefore your predicate is evaluated as

Date >= '1905-06-12'

which is probably not what you meant...

Put your date in single quotes.