Resources

Each guide explains one real, repeated temporal-reasoning decision, such as a counting convention, a calendar edge case, or a time-zone assumption, that shows up across several WhenMath engines rather than belonging to just one. See the Methodology page for the site-wide calculation rules these guides expand on.

• Business-Day Counting ConventionsBusiness Days

  Two people can both say "10 business days" and mean two different end dates, depending on whether the start date counts and which holiday list applies.

• Inclusive vs. Exclusive Date and Time RangesDate & Calendar Math

  A range from Monday to Friday can be 4 days or 5 days, and both counts are correct answers to slightly different questions.

• Why Adding a Month to Month-End Dates BreaksDate & Calendar Math

  January 31 plus one month cannot land on February 31, so every date system has to pick a rule for what happens instead, and the rules disagree.

• Calendar Months vs. Fixed-Day PeriodsDate & Calendar Math

  "Net 30" and "one calendar month" sound interchangeable but produce different due dates for all but a handful of start dates each year.

• Recurrence Drift: Why "Every Month on the 31st" BreaksRecurrence & Events

  A recurrence rule anchored to a specific day number silently skips or shifts in any month that day does not exist, and the shift compounds over a year.

• How Daylight Saving Transitions Distort Elapsed-Time MathTime Zones & Meetings

  Twice a year, a day does not have 24 hours, and clock-based elapsed-time math that ignores this can be off by exactly one hour.

• IANA Time Zones vs. Fixed UTC OffsetsTime Zones & Meetings

  A time zone is a place's ongoing set of clock rules, not a fixed offset from UTC, and treating it as one breaks calculations whenever DST changes.

• Finding Real Working-Hours Overlap Across Time ZonesTime Zones & Meetings

  Two people in different zones do not just have a time difference; they have an overlap window that can be small, awkward, or nonexistent.

• Why Deadline Buffers Need to Be Sized, Not GuessedDeadlines & Capacity

  A buffer that is too small does not protect the deadline, and a buffer that is too large wastes real calendar time that could have gone to the work itself.

• How Timecard Rounding Rules Change Paid HoursWork Hours & Shifts

  Rounding a clock-in or clock-out time is not neutral: the specific rule determines whether it favors the employer, the employee, or neither.

• What "Holiday" Means in a Business-Day CalculationBusiness Days

  A holiday calendar is a specific, dated list for a specific country and region, not a universal set of dates every calculation should assume.

• Leap Years and Leap-Day Handling in Date MathDate & Calendar Math

  February 29 exists in most but not all years divisible by 4, and a date calculation that gets the leap-year rule wrong is silently off by exactly one day.

• ISO Week Numbers vs. Plain Calendar WeeksDate & Calendar Math

  "Week 1" of a year does not always start on January 1, and two different week-numbering systems can disagree about which week a given date falls in.

• Fiscal Quarters vs. Calendar QuartersDate & Calendar Math

  "Q1" means January-March to most people, but it means whatever three-month block an organization's fiscal year starts on, which varies widely.

• UTC, Local Time, and Wall-Clock Time Are DifferentCore Time Math

  A clock reading, a timezone-qualified moment, and a fixed point on the universal timeline are three distinct concepts that get casually conflated as "the time."

• Why the Day of the Week for Any Date Is Fully DeterminedDate & Calendar Math

  Once a calendar system and one reference date are fixed, the weekday of every other date follows automatically, with no ambiguity or convention to choose.

• How Overnight Shifts Get Counted and Attributed to a DayWork Hours & Shifts

  A shift that starts at 10 PM and ends at 6 AM crosses a calendar date boundary, and different systems attribute that shift's hours to different days.

• Custom Workweeks: When Saturday-Sunday Is Not the WeekendBusiness Days

  A business-day calculation that assumes Saturday and Sunday are always the non-working days gets the wrong answer anywhere that uses a different workweek.

• Critical Path Basics: Why Some Delays Slip the DeadlineDeadlines & Capacity

  On a multi-phase plan, a delay in one phase may push the deadline back by that amount, or have no effect at all, depending on whether that phase has schedule slack.

• SLA Deadlines and Grace Periods Are Not the Same ClockDeadlines & Capacity

  An SLA response window and a grace period after expiration answer different questions, and confusing the two can misjudge whether a response was actually late.

• What an .ics Calendar Export Actually EncodesDeadlines & Capacity

  An exported calendar file has to make explicit choices about time zones and all-day events that a plain date or date-time value on a web page never had to make.

• Calendar Age vs. Exact Elapsed AgeAge & Life DatesInteractive

  The age you state on a form and the exact number of days you have been alive answer two different questions, and they only ever agree on your exact birthday.

• How a Leap-Day Birthday Is Handled in a Non-Leap YearAge & Life Dates

  Someone born on February 29 has no real birthday in three out of every four years, so any age or anniversary calculation has to pick an observed date instead.

• Why Timesheet Period Boundaries Must Be ExplicitWork Hours & Shifts

  A pay period stated as "the 1st through the 15th" is ambiguous about whether hours worked at the exact boundary belong to this period or the next one.

• What Makes a Cross-Timezone Meeting Rotation FairTime Zones & Meetings

  Always meeting at the same clock time seems neutral, but it silently makes the meeting inconvenient for the same person or team every single time.

• Why a Browser Timer Needs Anchor-Based MathTimers & Daylight

  A timer that just counts down from a starting number drifts or stalls the moment its browser tab is backgrounded, while one anchored to a real timestamp does not.

• Sunrise and Golden Hour Are Defined by Sun AngleTimers & Daylight

  Sunrise is not "6 AM plus an offset": it is the moment the sun crosses a defined angle below the horizon, which shifts daily and with latitude.

• Handling a One-Time Exception on a Regular ScheduleRecurrence & Events

  A weekly meeting that skips one week for a holiday is still "weekly"; the exception should be layered on the rule, not rewritten as a new one.

• Countdown and Count-Up Are the Same MathTimers & DaylightInteractive

  The same instant answers "how much longer" as a countdown and "how long has it been" as a count-up; a display doing only one looks wrong once the target passes.

• Blackout Dates Are Not the Same List as Public HolidaysDeadlines & Capacity

  A project blackout date is a separate, deliberately chosen exclusion list, not a subset or superset of public holidays.