is the "due date" not the deadline?
Two out of every seven years, April 15 falls on a Saturday or Sunday. For simplicity, we often refer to the next business day as the "due date" of the return or payment. But that is not technically correct. The actual due date of returns and payments is set by statute, and it remains the 15th of the month. Legally, where the actual due date falls on a weekend or holiday, returns may be filed and payments may be made on the next business day without penalty. However, interest computations, statutes of limitation, and extension periods all begin on the actual due date, April 15 for individual, calendar year taxpayers, even if it falls on a weekend or holiday.
These same rules apply in other areas where deadlines fall on weekends or holidays. The statute of limitations for proposing assessments and filing refund claims is normally four years from the date the return is filed or one year from the date of payment. Tax returns and payments for the current tax year that are filed before the original due date are treated as if they are filed on the original due date. So for a personal income tax or LLC 2005 return filed early, the original due date is still Saturday, April 15, 2006, and the four-year statute of limitations will expire Thursday, April 15, 2010. But if a taxpayer takes advantage of the weekend grace period, and actually files the return on Monday, April 17, 2006, then the four-year period ends Saturday, April 17, 2010, four years from the date the return was actually filed. Since April 17, 2010, falls on a Saturday, in this particular situation a claim or assessment notice will be timely if mailed on Monday April 19, 2010.
Where the deadline is not based on a specific date, but on a period of time like "six months" or "one year" then even more interesting problems arise. What happens if the six-month period begins on August 31? There is no February 31. The answer is that on the last day of February, six months have elapsed, so the payment or filing is due on February 28 (February 29 if it is a leap year). In the opposite situation, where the six-month period starts on February 28, the last day of the month, the notice is not late until September 1. It can be filed on August 30 or 31 and still be within six months of February 28. Unless, of course, that due date is a weekend or holiday, then it would be timely if mailed on the next business day as described above.
References: 18 Cal. Code Regs. Section 18566 (return due dates); Calif. Civil Code Section 11 (general rule that action performed on the next business day after a weekend or holiday is timely); Cal. Code of Civil Proc. Section 12 (Computation of time for performing acts required by law.) See also federal Internal Revenue Code Section 7503, PLR 6002104690A, 02/10/1960 (six months issue).