Having gone through PenFed myself, I can say that the rate is determined on the day when Tesla cashed the loan check. For example, if you have gotten an approval today with the rate of 1.49% but you didn't take delivery of the car until March, then PenFed will use the rate that is set for Mar as your rate for the entire loan. This is stated explicitly on the check that PenFed sent to me (the check is good for 60 days but the rate is determined when PenFed received the check back from Tesla).
This is what they told me as well, but I thought the check was good for 90 days?