Short answer:

• Best time: Early morning of the same day, before taking any food or water, after your morning vidhi/samayik.
• If missed: You may still take pachchakkhan any time before your single meal, but strictly before sunset.
• Not valid for today if taken after sunset; in that case it applies for the next day.

More detail (per tradition):

• Shwetambar: Take ekasana pachchakkhan in the morning (after prātaḥ-vidhi/pratikraman). Eat your one meal in a single sitting, before sunset. If you remember later, you may take the pachchakkhan just before that one meal, still before sunset.
• Digambar: Do pratyākhyāna for ekabhukti in the morning after devapūjā/saṅlekhana. The one meal is taken during the day, in one sitting, before sunset. If not done in the morning, take it before the meal itself, but not after sunset.

Essential points to keep:

• Duration: The resolve generally runs sunrise-to-sunrise.
• Purity of intent: Take pachchakkhan with right bhāva, specify “one meal, one sitting, before sunset,” and other niyams you follow (e.g., place, items avoided, boiled water rules per your tradition).
• If you have already eaten/drunk in a way that breaks ekasana, you cannot take pachchakkhan for ekasana for that same day.

Helpful references:

• Ekasanu & Biyasanu pachkhan audio/text
• Sample pachkhan wordings for different taps, including ekasana