Article — 45 Day Calculator
45 Day Calculator: Forward and Backward Date Math
The 45 day calculator finds the exact date 45 days from any start date, or 45 days before it. 45 calendar days equals 6 weeks and 3 days, or about 1.48 months. Under US federal law, the default reading of "45 days" is calendar days unless the rule says otherwise.
The 45-day window shows up in places that matter financially. The IRS gives investors exactly 45 calendar days to identify replacement property in a 1031 exchange. RESPA gives mortgage servicers 45 days to deliver an initial escrow statement. Mortgage lenders quote 45-day rate locks by default, and a growing number of B2B sellers use NET 45 invoice terms instead of NET 30 or NET 60. Each of those uses the same 45-day count, but the consequences of missing the deadline vary from a polite reminder to a six-figure tax bill.
What the 45 day calculator does
The calculator takes a start date, adds or subtracts 45 days, and returns the target date plus the day of the week. The default counts calendar days, which is how most US laws and contracts read "45 days" unless they explicitly say business days. Toggle to business days and the engine skips Saturdays and Sundays, which stretches 45 business days into about 63 calendar days, or roughly nine work weeks.
The result includes both the absolute date and a breakdown into weeks plus extra days. Picking January 1 returns February 15. Picking February 1 in a non-leap year returns March 18; in a leap year it returns March 17, because February has one extra day in the window. The calculator handles leap years automatically through the browser's Date API, so you do not need to track the rule manually.
The IRS paid out about 3.3 billion dollars in interest on delayed refunds in fiscal year 2020, much of it because pandemic backlogs pushed refund issuance past the 45-day window in IRC §6611(e). The IRS interest rate has run 7 to 8 percent annually since 2023.
45 days vs. 1.5 months
People say "45 days" and "1.5 months" as if they are interchangeable. They are not. 45 calendar days is always exactly 45 days. 1.5 calendar months can be anywhere from 44 to 47 days depending on which months you span. January 1 plus 1.5 months is mid-February. July 1 plus 1.5 months is mid-August, which is two more calendar days. For court filings, contract deliverables, and tax deadlines, count the days, not the months.
The cleanest mental model: 45 days is 6 weeks plus 3 days. Six weeks alone is only 42 days. Those three extra days are the difference between hitting a deadline and missing it.
45 calendar days vs. 45 business days
Calendar days count every day of the week, including Saturdays, Sundays, and federal holidays. Business days count only Monday through Friday and usually exclude federal holidays as well. 45 calendar days runs about 6.5 calendar weeks. 45 business days runs about 9 calendar weeks, since each week contains only 5 business days.
- 45 calendar days = 6 weeks + 3 days = about 1.48 months
- 45 business days ≈ 63 calendar days ≈ 9 weeks (subtract 1-3 days for federal holidays)
- Default in US federal law = calendar days, unless a statute says "business" or "working"
- 1031 exchange = calendar days, hard cutoff at midnight day 45
- NET 45 invoice = calendar days from invoice date
- RESPA 45-day notice = calendar days before the trigger event
- SEC and many regulator deadlines = business days, explicitly
The IRS 1031 exchange 45-day rule
The 45-day identification period under IRC §1031 is the harshest 45-day deadline in US tax law. A real-estate investor who sells investment property and wants to defer capital gains tax must identify replacement property in writing within 45 calendar days of the sale. The IRS does not extend the deadline for weekends, holidays, or hardship. Even during the early months of the COVID-19 pandemic, the IRS only granted limited relief through Notice 2020-23, and only for deadlines falling between April 1 and July 15, 2020.
Investors can use the Three-Property Rule (identify up to three replacement properties of any value), the 200% Rule (identify more than three, but the aggregate fair market value must be at most 200% of the relinquished property), or the 95% Rule fallback. Missing day 45 by a single hour disqualifies the exchange and turns a tax-deferred transaction into a taxable capital gain.
If day 45 lands on a Sunday or a federal holiday, the deadline does not roll forward. Identification must be in the hands of the qualified intermediary by 11:59 PM local time on day 45. There is no extension procedure.
NET 45 invoice terms
NET 45 means the invoice is due 45 calendar days after the invoice date. It is a middle-ground payment term between NET 30 (small-business default) and NET 60 or NET 90 (large enterprise default). NET 45 has grown in popularity among mid-market buyers who want a longer float without forcing vendors into the cash-flow strain of NET 60.
The deadline counts from the invoice date printed on the document, not from the day the buyer receives it or processes it. A few contracts use "NET 45 from receipt," which shifts the start date but not the duration. If the contract is silent, the invoice date wins.
Mortgage rate locks and 45-day windows
Most US mortgage lenders offer a 45-day rate lock as the default option. That gives the borrower 45 calendar days to close the loan at the locked rate without paying an extension fee. Shorter locks (15, 21, or 30 days) usually carry a small rate discount. Longer locks (60, 75, or 90 days) cost extra, typically 0.125 to 0.25 of a discount point.
The 45-day window is calibrated to the typical mortgage closing timeline. From application to closing, a conventional purchase loan in 2025 averages about 42 to 45 days, which fits inside the standard lock window with little margin. A refinance closes faster, often in 30 to 35 days, which is why some lenders offer cheaper short-lock pricing for refinances.
If your closing slips past day 45, ask the lender about a lock extension before the lock expires. Same-day extensions are usually cheaper than relocking at the prevailing rate, especially in a rising-rate market.
RESPA 45-day notice rules
The Real Estate Settlement Procedures Act (RESPA), implemented through Regulation X under the Consumer Financial Protection Bureau, uses 45-day windows in several servicer obligations. A servicer must send notice at least 45 days before charging a borrower for force-placed insurance on a property. The initial escrow account statement must be delivered no later than 45 days after settlement. Loss-mitigation applications submitted 45 or more days before a scheduled foreclosure sale trigger the full evaluation procedure under 12 CFR 1024.41.
Common 45-day calculation mistakes
Three things trip people up. First, mixing calendar and business days. The IRS 1031 deadline is calendar; SEC business-day deadlines look similar in length but compute differently. Second, treating 45 days as 6 weeks. It is 6 weeks plus 3 days, and those extra three days can shift a deadline across a month boundary. Third, time zones for federal deadlines. The IRS counts the day in the time zone where the filing or identification is delivered, not the taxpayer's home zone.
45 calendar days = 6 weeks + 3 days45 calendar days ≈ 1.48 months45 business days ≈ 63 calendar days45 business days ≈ 9 work weeks1031 identification = 45 calendar days, no extensions