Śaiśirīya Śikṣā 16

16

hrasvo dīrghaḥ pluta iti svarāḥ kālena saṃjñitāḥ
sandhyakṣarāṇāṃ kālaśca dīrghaḥpluto na mātrikaḥ

Short, long, and protracted: thus vowels are designated by duration. / And the duration of diphthongs is long [or] protracted; it is not one mora.