After I shared one solution about oracle, iis and sysdate problem, I got another similar error. This time, the function was getting the paramater sysdate with time added. I noticed this because the function was calculating default insterest of a price, and each hour it was increasing the total amoun.
After many tries, I removed NS_LANG key from reg entry of Oracle. See below.
If it doesnt work, try to add your language and setting, Turkish in my case.