Date-Only Range Construction
This tool evaluates dates sequentially between a specified start date and end date to calculate day totals. The input fields are constrained to the range from January 1, 2020, to December 31, 2027.
By default, the start date is initialized to the device's local calendar date, with the end date preselected to 30 days later. If the current date falls outside the supported range, it is clamped to the boundary dates of the range.
Input values are validated as YYYY-MM-DD strings, rejecting invalid calendar entries such as 2026-02-29. Date additions and day-of-week checks rely on UTC day numbering, not to convert or display inputs in Coordinated Universal Time, but to process dates consistently without time components and prevent timezone offsets from shifting Japanese date keys to previous days.
A date displayed as 2026-09-22 matches 2026-09-22 internally against Japanese holiday records.
If the start date is later than the end date, the tool does not halt with an error; instead, it automatically sorts the dates from earliest to latest and displays an alert indicating that reordering occurred.
For instance, entering 2026-09-25 as the start date and 2026-09-18 as the end date calculates the span from September 18 to September 25.
Because it does not count backward based solely on input field labels, review the notification to ensure the evaluated range matches your intent.
Start Date Inclusion and Weekend or Holiday Exclusions
The option "Include start date in count" is enabled by default. When enabled, counting begins on the earliest date after sorting; when disabled, counting begins on the following day, continuing through the end date.
The end date is always included in the target range, and there is no standalone setting to exclude only the end date. If the exact same date is chosen for both the start and end dates while start date inclusion is disabled, the target count resolves to 0 days.
Saturdays, Sundays, and Japanese holidays can each be excluded using dedicated toggle switches. Configurations such as treating Saturdays as working days while excluding Sundays are supported.
However, the interface does not offer recurring weekly exclusion settings for other specific days of the week, such as Mondays or Tuesdays.
Workplaces observing those schedules must register each affected date manually as a custom holiday.
Each date is assigned a day-of-week index ranging from Sunday (0) to Saturday (6), and dates matching enabled exclusion rules are omitted.
If holiday exclusion is turned off, national holidays falling on weekdays are counted as business days. Nevertheless, if such a date simultaneously matches an active exclusion rule for Saturdays, Sundays, or custom holidays, it remains excluded by the other condition.
"Business days" here does not represent a statutory legal definition, but rather the remaining days after subtracting the user-selected criteria.
Official Holiday Dataset from 2020 to 2027
Japanese public holidays incorporate published data from the Cabinet Office's "About Public Holidays" CSV spanning 2020 through 2027.
Future Vernal Equinox and Autumnal Equinox Days are not estimated using mathematical approximations, nor is the timeline constructed solely from standard Monday rules.
Consequently, dates rescheduled for the Tokyo Games in 2020 and 2021—Marine Day, Sports Day, and Mountain Day—are retained exactly as officially designated.
For 2026, the dataset includes the May 6 holiday, Respect for the Aged Day on September 21, the September 22 holiday falling between two holidays, and Autumnal Equinox Day on September 23.
For 2027, Vernal Equinox Day on March 21 and the March 22 holiday are registered. Days designated simply as "Holiday" in official CSV records are displayed as "Holiday" on screen without assigning custom substitute or citizens' holiday classifications.
Entering dates earlier or later than the supported range halts calculation rather than extrapolating from known patterns. Because vernal and autumnal equinox dates for the following year are announced in the Official Gazette in February of the preceding year based on calculations by the National Astronomical Observatory of Japan, unannounced years cannot be permanently predicted with certainty.
Holiday data for 2028 onward requires site updates following official announcements, and legal revisions or unscheduled holidays will not reflect automatically prior to data updates.
Custom Holidays and Overlapping Exclusion Logic
Corporate year-end closures, founding anniversaries, and retail store closures can be added individually through "Add custom holiday".
Registered dates must also fall within the 2020 to 2027 span. Duplicate dates are prevented, and entered dates appear as sorted badges that can be removed individually using their trash button.
Custom holidays lack individual toggle switches and remain excluded as long as they are listed. If a Saturday, a public holiday, and a custom holiday coincide on the same date, that day is never subtracted two or three times.
Exclusion is evaluated once per calendar date using a logical OR condition, reducing the aggregate business day count by exactly one day.
The exclusion summary list details the date, day of the week, and primary exclusion reason. When a row matches multiple criteria and includes an official public holiday, the holiday name takes display precedence; if it is not an official holiday but matches a custom entry, it displays as "Custom Holiday", and otherwise displays as "Weekend".
This indicates the representative label shown in the list rather than duplicate deductions.
Added dates persist exclusively within React component state, without being saved to browser local storage or synced to user accounts.
Reloading the page resets the form to its initial state. Recurring schedules such as the second Tuesday of every month, CSV imports, and multi-device synchronization are not supported.
Interpreting Counts, Weekday Breakdowns, and Exclusion Lists
The top of the results section displays "Working / Business Days", "Excluded / Holidays", and "Total Days in Period". Total days reflects the span after applying start date inclusion settings.
Business days represents that total minus dates matching any exclusion rule, with multiple matching conditions counting as only a single deduction per day.
As a concrete example, take the period from September 18 to September 25, 2026, including the start date and excluding Saturdays, Sundays, and Japanese holidays.
The total target span is 8 days. Excluding 5 days—Saturday the 19th, Sunday the 20th, Respect for the Aged Day on the 21st, the holiday on the 22nd, and Autumnal Equinox Day on the 23rd—leaves 3 business days: the 18th, 24th, and 25th.
Registering September 22 as a custom holiday keeps excluded days at 5 because it is already excluded as a public holiday.
The weekday breakdown indicates how many times each day of the week occurs across the evaluated span. It counts all target days before exclusion rather than reflecting solely surviving business days.
To check which specific days were removed, consult the "Excluded Dates & Holidays in Period" table below. Weekday public holidays omitted from exclusion rules will not appear in this list and remain counted among business days unless caught by other rules.
Displayed counts update whenever configuration switches change. Verifying total days, excluded counts, and daily exclusion reasons together before copying figures helps catch unintended omissions in start date inclusion or company holiday lists.
Operational Scope and Boundaries for Deadline Planning
This tool functions as a date counter to check available workdays between two calendar endpoints under configurable conditions. It does not perform backward scheduling such as calculating a date five business days away, nor does it support end date exclusion, half-day shifts, working hours, headcount, person-months, or workload estimation.
It also does not determine legal deadlines, banking settlement dates, or administrative filing cutoffs automatically.
Public holidays do not always coincide with institutional closures. Bank holidays, year-end office closures, municipal counter operating schedules, client shutdown periods, and overseas holidays cannot be determined solely through Cabinet Office public holiday data.
Known closure dates can be added as custom holidays, but these entries do not persist and must be re-entered upon returning to the tool.
Contracts and administrative procedures define business days, first-day inclusion, last-day inclusion, and weekend or holiday cutoff extensions differently depending on governing statutes and agreements.
The configuration switches here will not automatically conform to every institutional framework. Match critical deadlines directly against relevant contracts, governing agencies, or client operational calendars.
Furthermore, the built-in holiday data represents a static snapshot through 2027. Even within the supported range, official corrections or statutory shifts might not appear prior to site updates.
Rather than treating results as certified documentation, use them as an interactive reference to visualize applied constraints and verify day counts.