Can a person make an intention to keep a Fast after Sehri time has ended?

The intention to observe a Fardh, Sunnah or Nafl fast should be made at night before Dawn (Subah Saadiq). However, if one did not make an intention before Dawn to observe the fast, one may still make an intention to observe a Fardh, Sunnah or Nafl fast until Dahwatul Kubra (close to midday) provided that one did not engage in any acts that would nullify the fast after Dawn.

Dahwatul Kubra is the middle time between Dawn and Sunset. For example, on 20th March 2024, dawn is at 4:40 AM and sunset is at 6:08 PM. The total time between Dawn and Sunset is 13 hours 28 Minutes so the middle time between them is 6 hours and 44 minutes.

Accordingly, if one did not make an intention to observe a Fardh, Nafl or Sunnah fast before Dawn and one wishes to make an intention to observe it after Dawn, one may do so until 11:24am provided that one did not engage in any acts that would nullify the fast after Dawn. (Lubaab 1/162-Shaami 2/377-)

N.B. In terms of making intention of Qadha fasts, the intention must be made before Dawn. An intention for a Qadha fast after Dawn is invalid.

And Allah Ta’ala Knows Best