Hayanaratna 1.241

Now the finding of the nata (time from midday or midnight) and the rest is told in the Paddhatichintamani. Its author is a royal swan playing in the lake of all the siddhantas. He is a lion among the masters of skill in ganita (maths of the planets). He is expert in the speech of the Serpent. First, finding the nata and the unnata (time from midnight). At night, the ghatikas gone or left of the night, added to the half-day, are the nata. They are the western and the eastern nata in turn. By day, the half-day less the time gone since sunrise is the eastern nata. The time gone since sunrise less the half-day is the western nata.

Simple meaning: Time from midday: by day, the gap between now and midday; at night, the night gone or left plus half the day.