Modulo on long numbers

Is there any bug in the Java Script Modulo?

I have to add to my form validation of an account number. The problem is that it is validating by modulo of 30 digits number. I did everything right but it came out that my accounts were wrong. Each one. So I checked one by one and looks like modulo is broken after reaching 24 digits.

Just check this validation script:

app.alert("10 digits - "+1000000000%10);
app.alert("20 digits - "+10000000000000000000%10);
app.alert("21 digits - "+100000000000000000000%10);
app.alert("22 digits - "+1000000000000000000000%10);
app.alert("23 digits - "+10000000000000000000000%10);
app.alert("24 digits - "+100000000000000000000000%10);

Any idea how to bypass that? I can't do anything else besides modulo by 97 to validate bank account.