29 C.F.R. § 4000.43 – How do I compute a time period?

(a) In general. If you are computing a time period to which this part applies, whether you are counting forwards or backwards, the day after (or before) the act, event, or default that begins the period is day one, the next day is day two, and so on. Count all days, including weekends and Federal holidays. However, if the last day you count is a weekend or Federal holiday, extend or shorten the period (whichever benefits you in complying with the time requirement) to the next regular business day. The examples in paragraph (d) of this section illustrate these rules.
(b) When date is designated. In some cases, our regulations designate a specific day as the end of a time period, such as “the last day” of a plan year or “the fifteenth day” of a calendar month. In these cases, you simply use the designated day, together with the weekend and holiday rule of paragraph (a) of this section.
(c) When counting months. If a time period is measured in months, first identify the date (day, month, and year) of the act, event, or default that begins the period. The corresponding day of the following (or preceding) month is one month later (or earlier), and so on. For example, two months after July 15 is September 15. If the period ends on a weekend or Federal holiday, follow the weekend and holiday rule of paragraph (a) of this section. There are two special rules for determining what the corresponding day is when you start counting on a day that is at or near the end of a calendar month:
1. (1) Special “last-day” rule. If you start counting on the last day of a calendar month, the corresponding day of any calendar month is the last day of that calendar month. For example, a three-month period measured from November 30 ends (if counting forward) on the last day of February (the 28th or 29th) or (if counting backward) on the last day of August (the 31st).
2. (2) Special February rule. If you start counting on the 29th or 30th of a calendar month, the corresponding day of February is the last day of February. For example, a one-month period measured from January 29 ends on the last day of February (the 28th or 29th).
(d) Examples—(1) Counting backwards. Suppose you are required to file an advance notice of reportable event for a transaction that is effective December 31. Under our regulations, the notice is due at least 30 days before the effective date of the event. To determine your deadline, count December 30 as day 1, December 29 as day 2, December 28 as day 3, and so on. Therefore, December 1 is day 30. Assuming that day is not a weekend or holiday, your notice is timely if you file it on or before December 1.
(2) Weekend or holiday rule. Suppose you are filing a notice of intent to terminate. The notice must be issued at least 60 days and no more than 90 days before the proposed termination date. Suppose the 60th day before the proposed termination date is a Saturday. Your notice is timely if you issue it on the following Monday even though that is only 58 days before the proposed termination date. Similarly, if the 90th day before the proposed termination date is Wednesday, July 4 (a Federal holiday), your notice is timely if you issue it on Tuesday, July 3, even though that is 91 days before the proposed termination date.
(3) Counting months. Suppose you are required to issue a Participant Notice two months after December 31. The deadline for the Participant Notice is the last day of February (the 28th or 29th). If the last day of February is a weekend or Federal holiday, your deadline is extended until the next day that is not a weekend or Federal holiday.