system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can you calculate a day for a planet if it is tidally locked?
gpt
If a planet is tidally locked, then it always faces the sun with the same face. So if you define a day as the amount of time it takes the planet to make one rotation about its axis, it would be equal to its year.

 If you define a day as the time between one sunup and the next, it doesn't have them.