I just entered the formula in a spreadsheet using the parameters you gave before, and that's the value I got.

The curve as given in the datasheet I linked to is for a thermistor with the same B-coefficient as yours, and it shows about 18k at 0°C.

If your data sheet says 30k at 0°C, it must have a different B-coefficient. Or they calculated the value based on the B-coefficient, but didn't actually measure it: if I enter 30000 in the formula above for R, it gives me a temperature of -1°C.