How do I calculate a visit window?

Add the visit's study-day offset to the anchor date to get the target date, then subtract the days allowed early and add the days allowed late. Under the Day 1 convention, Day N is the anchor plus N minus 1 days; under the Day 0 convention, Day N is the anchor plus N days.

Does this calculator skip weekends or holidays?

No. It counts calendar days, which is how most protocols define windows. If your protocol uses business days, adjust manually.

What is the difference between Day 0 and Day 1?

Under the Day 1 convention (used by CDISC SDTM study days) the anchor date is Day 1 and there is no Day 0. Under the Day 0 convention the anchor date is Day 0. The same visit lands one day apart, so check your protocol. See our guide [How to calculate visit windows](/resources/how-to-calculate-visit-windows).

Free tool

Visit window calculator

Enter the anchor date, the visit's study day and its window. Get the target date and the first and last allowed dates. Calendar days, nothing skipped.

Target date

Thu, Jan 15, 2026

Window opens: Mon, Jan 12, 2026
Window closes: Sun, Jan 18, 2026
Days after anchor: 14
Dates in window: 7

Step 1
Enter the anchor date
Pick the date the schedule counts from, such as randomization or first dose.
Step 2
Choose Day 1 or Day 0
Choose whether the protocol calls the anchor date Day 1 or Day 0.
Step 3
Enter the visit day and window
Enter the visit's study day and how many days early and late are allowed.
Step 4
Read and copy the result
The calculator shows the target date and the window open and close dates. Copy the result into your notes.

Worked example: Day 15 with a three-day window

Illustrative schedule: the anchor is 1 January 2026 and the protocol calls it Day 1. The target visit is Day 15, with three days allowed early and three late. Day 15 is fourteen days after the anchor, so the target is 15 January. The allowed dates run from 12 January through 18 January, inclusive: seven dates.

Change only the convention to Day 0 and the same numeric visit day lands on 16 January. That one-day difference is why staff must read the schedule footnote instead of choosing a convention from memory. The calculator performs date arithmetic; it does not interpret the sponsor’s protocol language or approve a date.

Assumptions and limits

The tool counts calendar days. Weekends and public holidays are included, and a clinic closure does not change the protocol window. Use separate scheduling judgment to choose a bookable appointment. If your protocol defines business days, a variable anchor, time-of-day requirements or an assessment-specific window, this result needs additional review.

Enter whole-number study days and nonnegative early/late allowances. Under the Day 1 convention there is no Day 0; negative study days refer to days before the anchor. Check the anchor’s actual date, not the appointment that was originally planned. A delayed qualifying event can change later targets when the protocol counts from that event.

Use the result without losing the protocol context

Copy or export the result with its inputs so another staff member can reproduce the calculation. A date without the convention and anchor is not enough. Print the result for a planning discussion if useful, but file clinical scheduling decisions in the appropriate study record rather than relying on this public calculator.

Read the visit-window guide for asymmetric allowances and examples. If the actual visit falls outside the permitted dates, document what happened and follow protocol and reporting requirements. Do not change the anchor merely to make a late visit appear in window.

Questions people ask.

Answers to the questions research sites ask most.

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