Pachakhan means self observant oath. It means you have committed to limit yourself with total food and water intake or limited intake as per local timings. Essentially Pachakhan is commitment for doing Jain way doing Tapashraya.
For examples, a) In Upvas, a person doesn’t eat or drink anything except boiled water. Boiled water can be drunk from Porsi to sunset Only. Usually, Pachakhan is available in Panchang books for major Indian cities. b) Sadhporsi Pachakhan means don’t eat or drink anything until 1.5 Prahar time after the sunrise.