This refers to the time that the Son of Man will return. Alternate translation: "that day or that hour that the Son of Man will return" or "the day or the hour that I will return" (See: [[rc://en/ta/man/translate/figs-explicit]])
These words specify some of those who do not know when the Son of Man will return, different from the Father, who does know. Alternate translation: "no one knows—neither the angels in heaven nor the Son know—but the Father" or "neither the angels in heaven nor the Son know; no one knows but the Father" (See: [[rc://en/ta/man/translate/figs-ellipsis]])
It is best to translate "Father" with the same word that your language naturally uses to refer to a human father. Also, this is an ellipsis, stating that the Father knows when the Son will return. Alternate translation: "but only the Father knows" (See: [[rc://en/ta/man/translate/figs-ellipsis]])