Visit Schedule and Window Calculator.

Every visit has two numbers that are easy to mix up: the days elapsed since the reference date, and its study day label. In the CDISC SDTM standard the reference start date is Study Day 1, the day before is Day −1 and there is no Day 0[2]:

On or after the reference date: study day = days elapsed + 1
Before the reference date: study day = minus the days before

That is why Week 4 is Day 29. A visit 28 days after the reference date is labelled 28 + 1, so Day 29. The mRNA-1273 phase 3 trial gave two injections 28 days apart, on days 1 and 29[3]. Other trials call the reference date Day 0. The ChAdOx1 nCoV-19 report defines Day 0 as the day of vaccination[4], so labels equal elapsed days and Week 4 is Day 28.

The trap is typing a study day where the protocol means elapsed days. With Day 1 numbering, a visit entered as Day 28 lands one day before Week 4. The calculator shows the study day and the elapsed days side by side. Switching the numbering relabels visits entered in days, weeks or months without moving them, while visits entered as study days keep their numbers and move by one day. A window that crosses the reference date skips Day 0: three days before Day 1 runs Day −3, −2, −1 and 1.

“Month 6” can mean six calendar months or a fixed number of days. Six calendar months span 181 to 184 days depending on the start date, against 180 days for six 30 day months. By default the calculator counts calendar months from the anchor date and keeps the day of the month. When that day does not exist, it uses the last day of the month:

31 Jan 2027 + 1 month = 28 Feb 2027
31 Jan 2028 + 1 month = 29 Feb 2028
31 Jan 2027 + 3 months = 30 Apr 2027

Each month is counted from the anchor, never by adding one month at a time, so Month 2 after 31 January is 31 March, not 28 March. Python dateutil and SAS INTNX with the same alignment use this rule, so a statistician can reproduce the dates[5][6].

If your protocol defines a month as a fixed number of days, choose 28 or 30 days. The SPIRIT 2013 explanation paper quotes a protocol whose 2, 4 and 6 month follow ups were 56, 112 and 168 days after randomisation, which are 28 day months[1]. When the protocol gives day or week numbers, enter those instead.

A visit window is the range of dates around a target when the visit is allowed to happen. ICH E6(R3) notes that acceptable ranges for protocol provisions “can reduce the number of deviations or in some instances the requirement for a protocol amendment”, provided they do not affect participant safety or scientific validity[7]. FDA draft guidance says intentional departures should be rare because the protocol should include appropriate flexibility, naming “reasonable visit windows” as an example, and asks sponsors to assess which elements, visit windows included, can be made more flexible[8]. ICH M11, the structured protocol template, states: “Allowable windows should be stated for all visits and procedures”[9].

What happens when a visit is out of window

A protocol deviation is any change, divergence or departure from the study design or procedures defined in the protocol[10]. Under ICH E6(R3) the investigator should document all protocol deviations, and the sponsor sets trial specific criteria for which ones are important[7]. FDA’s draft guidance lists small deviations from protocol specified visit windows among its examples of deviations that are not important, and lists failure to collect data for primary or secondary endpoints as important[8]. Deviations are common: a benchmark of 187 protocols found a mean of 75 deviations in phase II and 119 in phase III protocols, involving nearly one third of the patients in those trials[11].

SPIRIT, the reporting guideline for trial protocols, asks for a time schedule of enrolment, interventions, assessments and visits, with a schematic diagram “highly recommended”[12]. The 2025 update asks protocols to list target time points and acceptable time windows in that diagram, for example 30±3 days[12]. A concise timeline helps guide the conduct of a trial and lets reviewers judge participant burden and feasibility[13]. ICH E6(R3) also expects the protocol to describe the schedule of events[7].

The calendar file follows the iCalendar standard, RFC 5545[22]. Each visit is an all day event on its target date, with the window, study days and flags in the description. You can export one multi day event per visit covering the whole window instead. All day events carry no time zone, so each one sits on the same calendar date for everyone[22]. Events are marked as free time so they do not block your calendar. Each event has a random ID that stays the same while the page is open. Whether a calendar app updates or duplicates events when you import a file again depends on the app.

The CSV follows RFC 4180[23], uses ISO dates such as 2026-11-30 so day and month cannot be swapped, and starts with a byte order mark so Excel reads it as UTF-8[24]. Text you type that starts with an equals, plus, minus or at sign gets a leading apostrophe so a spreadsheet does not run it as a formula[25]. Use a participant study code, never a name, because calendar files are often synced to cloud services.

• Dates only, no times. Windows such as sampling within 10 minutes of a dose need date and time arithmetic, which this tool does not do.
• Fixed schedules only. Cycles that depend on events, such as dose delays or treatment until progression, are not modelled. Fixed cycles can be entered in days.
• No holidays or site closures. Weekend flags are the only calendar check.
• One participant at a time. Nothing is saved, so export the schedule before you leave the page. To run schedules for every participant, see the WeGuide Program Builder.
• Not a system of record. The approved protocol defines the schedule. Check every date against it.