Rosh Hashanah occurs 163 days after the first day of Passover.
The two holidays are separated by a fixed interval in the traditional Hebrew calendar: Passover begins on 15 Nisan, while Rosh Hashanah begins on 1 Tishrei. Although the Hebrew calendar is lunisolar and its months vary in length, this relationship produces the familiar 163-day interval.
Passover marks the Exodus from Egypt and begins the month of Nisan, which is the first month for many religious and historical calculations. Rosh Hashanah begins Tishrei, the seventh month in that numbering, and marks the civil new year. Thus the same calendar has several “new years” serving different purposes.
The number is easy to confuse with the Gregorian date gap, which changes from year to year because the Hebrew and Gregorian calendars do not align permanently. It also should not be mistaken for the number of days between the first day of Passover and the eve of Rosh Hashanah; the standard statement counts from the first day of Passover to Rosh Hashanah itself.