The problem in your code is in the date part. Becuase there is no 18. month.
if you set a varialbe which tooks your date first, the problem will be solved.
Before doing this, first set your day month and year of your fitro object
The code below works.
DateTime FinalDate=DateTime.Now;//your fitro object date will be here
string xdate = String.Format(" Data <= '{0}' ", FinalDate.ToString("yyyy-MM-dd") + " 23:59:59");
Text = xdate;