When you submit this form, the server will work out the values of Ra, Rb and C.

With this particular circuit, t1 must be bigger than t2.

This is because the output is high (t1) whilst the capacitor is charging, it charges through Ra and Rb.

The output is low (t2) whilst it is discharging, it discharges through Rb only.

Therefore it always takes longer to charge than discharge.