How notice periods are counted
"Two weeks notice" in the US is a convention, not a law: 14 calendar days from the day you tell your manager, with your last day landing on a business day. If the 14th day is a Saturday, most people make Friday the last day. Some employers count ten business days instead, which lands in the same week. Contracts and executive agreements may specify 30, 60 or 90 days; those are almost always calendar days unless the agreement says otherwise.
At-will employment means neither side is legally required to give notice in most states. Giving it anyway protects references, PTO payout in states that require it, and eligibility for rehire.
Start dates
A new employer will usually ask for your earliest start. The honest answer is your last day plus whatever break you want plus one business day, which is what the calculator shows. Starting on a Monday is common but not required; many companies run orientation on specific days, so confirm before you commit.
Federal holidays used
New Year's Day, Martin Luther King Jr. Day, Presidents Day, Memorial Day, Juneteenth, Independence Day, Labor Day, Columbus Day, Veterans Day, Thanksgiving, Christmas. When a fixed-date holiday falls on a weekend the observed weekday is used. Your employer may not observe all of these; switch the holiday setting off to count every weekday.
For recruiters
When a candidate accepts on a Tuesday with two weeks notice and wants a week off, the realistic start is three weeks out, the Monday after. Use the business-days tool to set expectations with the hiring manager before the offer letter is written, and to count the days a contractor will actually bill in a short engagement.