This site explains that there are 4 rules that would cause Rosh Hashannah to be postponed 1 or 2 days after the date of Molad Tishrei. You can view all 4 rules on the web site, but rule 4 states:
4) If molad Tishri following a leap year falls on Monday at 15 hr 589 hq (9:32:43 1/3 AM) or later, Tishri 1 is delayed by one day for the following reason:
Molad Tishri of the leap year occurred on or after Tuesday at 18hr (noon) Therefore Tishri 1 of that leap year was delayed one day by (2 - Molad Zaken rule) and one more day by (1 - Lo Ad"u Rosh rule) This would make that leap year too short (382 days) Note that this delay would now cause Tishri 1 to fall on a Tuesday and that will never cause (1) to trigger a further delay
I have designed an Excel sheet with some Visual Basic programming to list the dates of Rosh Hashannah and tell me the postponement rules (if any) that were applied. I have listed all years from 5775 until 5900. I have not been able to locate any instance where this rule has been applied. Granted, I may have a logic error in my program, so perhaps, it exists after all.
Would anyone know when this rule was last applied and when the next occurrence would occur? Also, if you know of a quick way to determine this programmatically (VNA or psudeu code), I'd really appreciate that.
While I'm at it - Shana Tovah to all.