The only measurable thing you have with you is your body, but you won't want to rely on it.

Certain bodily functions happen at a generally dependable rate, at least over the course of a year. Hair growth is one, though accurate measurement may prove difficult. Your pulse is another stand-out candidate, particularly because it is discretely countable. This isn’t going to always be accurate, however, since the gravity and atmospheric makeup of your new planet may not be equal to Earth and could feasibly affect your resting heart rate.

With a significant portion of your concentration you might be able to track your pulse, but as soon as you become heavily active it will become a less stable timepiece. You will also lose track of large swaths of time when you fall asleep. You may be inclined to simply assume it was an 8 hours (or your equivalent back-of-the-envelope heartbeat conversion) rest, but the stress of your situation and the differing solar cycle are almost guaranteed to cause a wide array of fluctuations that lead to multiple hours of error a day.

Constructing a physical timepiece is a necessity, but it suffers from the same core problem as manual tracking: your base units of measurement can’t be guaranteed to be constant. Even if your timekeeping was only off by thirty minutes a day, your estimation of when the gate reopens would be off by more than a week.

I strongly recommend you build base camp at the foot of that gate and never let it leave your line of sight.

The only measurable thing you have with you is your body, but you won't want to rely on it.

Certain bodily functions happen at a generally dependable rate, at least over the course of a year. Hair growth is one, though accurate measurement may prove difficult. Your pulse is another stand-out candidate, particularly because it is discretely countable. This isn’t going to always be accurate, however, since the gravity and atmospheric makeup of your new planet may not be equal to Earth and could feasibly affect your resting heart rate.

With a significant portion of your concentration you might be able to track your pulse, but as soon as you become heavily active it will become a less stable timepiece. You will also lose track of large swaths of time when you fall asleep. You may be inclined to simply assume it was an 8 hours (or your equivalent back-of-the-envelope heartbeat conversion) rest, but the stress of your situation and the differing solar cycle are almost guaranteed to cause a wide array of fluctuations that lead to multiple hours of error a day.

Constructing a physical timepiece is a necessity, but it suffers from the same core problem as manual tracking: your base units of measurement can’t be guaranteed to be constant or accurate. Even if your timekeeping was only off by thirty minutes a day, your estimation of when the gate reopens would be off by more than a week.

I strongly recommend you build base camp at the foot of that gate and never let it leave your line of sight.