Time to vest calculation

Hope you’re doing well. We’re trying to recalculate the weighted average time to vest per what is shown on Carta’s support site (https://support.carta.com/s/article/calculates-expected-term). Over here, it says the weighted average time to vest is equal to the quantity vested times the time to vest however we couldn’t find a formula for the time to vest. Could you please let me know how time to vest is calculated? Thank you!