Saat-e Tahvil is the exact moment at which the sun moves into the section of the sky to begin spring.
Saat-e Tahvil this year is 1:14:27 a.m. Eastern Time in the United States Tuesday, March 20. In the Central Time Zone, that is 12:14:27 a.m. or less than 15 minutes into the First of Farvardin, the first day of the New Year under the Persian Solar calendar.
Farther west that will be 11:14:27 p.m. in the Mountain Times Zone and 10:14:27 in the Pacific Time Monday night.
In Iran, citizens will need to get up early on the holiday to mark Saat-e Tahvil at 8:44:27 a.m., Tuesday.
Because of the way the Gregorian Calendar handles leap years, Saat-e Tahvil can come long before or long after the March 21 date commonly noted as the first day of spring. But the Persian solar calendar has a much more complicated way of calculating leap years. The practical result of that complexity is that Saat-e Tahvil always falls within 12 hours of the midnight that divides the last day of the old year on the Persian calendar and the first day of the new year.
But that leap year calculation also means that your birthday in the coming 12 months has also shifted one day. For a fuller explanation of the Persian calendar and why your birthday has moved, see the story on the calendar that will run in our special Now Ruz issue next week.