Dates, BCE and fuzzy dates

Timescrolly can date something to the second, or to billions of years, and it can say a date is only roughly known rather than pretending otherwise. This article covers precision, calendars, BCE dates, uncertainty, spans, and how the viewer displays all of it.

How much of a date you actually know

A date field only asks for what you know. A year on its own is a complete date; adding a month, then a day, then a time (through add time, once there is a day) narrows it further, but you're never required to fill in more than that. Leaving a field blank drops everything finer than it too — a year with no month given isn't secretly read as January 1st, it's read as covering the whole year.

The Advanced switch

A date field starts out plain: a Gregorian year, month and day, in the current era. The Advanced switch beside the field's label adds everything else — the Calendar picker, the Era for Gregorian and Julian dates, and the uncertainty described below. It turns itself on when the timeline's own span, or the date you are editing, already needs it, such as a BCE year or a date in another calendar.

Choosing a calendar

With Advanced on, the Calendar picker at the top of the date field chooses how you write the date:

• Gregorian is today's calendar projected backwards, including into periods before it existed. An Era choice sets how the year is numbered: CE, BCE (no year 0: 1 BCE is directly followed by 1 CE), astronomical year numbering (which does have a year 0 and counts negatively before it), or a regional count — Holocene, Buddhist, Minguo, or a Japanese era (Meiji, Taishō, Shōwa, Heisei, Reiwa).
• Julian has the same eras, and can be set to begin its year on 25 March, for sources such as English dates before 1752. Those read like 1731/32.
• Hijri (the tabular Islamic calendar, which can differ by a day or two from an observed date), Hebrew, Coptic, Ethiopian, French Republican (years 1–14 only) and Persian (Solar Hijri, years 1178–1633 only) each count their own years and name their own months. There is no era to choose.
• Before present counts Julian years back from 1950, in years, thousands (ka), millions (Ma) or billions (Ga), for dates far enough back that a calendar year stops being useful.
• After present counts forward from 1950 the same way, in thousands (ka), millions (Ma) or billions (Ga) of years — up to 14 billion — and reads like 4.5 Ga AP. It has no single-year unit, because a year-precise future date is a Gregorian one; Gregorian dates end at the year 285,000,000, so anything later is written this way.

A date is kept in the calendar you wrote it in and shown that way, not converted to Gregorian.

Saying how sure you are

With Advanced on, add uncertainty under any date adds a stated margin: an amount and a unit, from seconds up to years for a date in any calendar, or years up to billions of years for a Before present or After present one. It stacks on top of precision rather than replacing it — a year-only date is already read as spanning that year, and uncertainty widens it further. For a Before present or After present date, the number of decimals you type already implies a precision on its own: 2.5 Ma reads as 2.45–2.55 Ma, shown as a hint under the field until you add uncertainty explicitly. How the viewer draws it is under Seeing how sure a date is.

Spans and ongoing periods

A period has both a Start and an End, or Ongoing if it hasn't finished. Because either end can carry its own precision, a period known only to the year can still contain events known to the day — nothing requires the two to line up.

Counting from an epoch

A timeline can count from an event of its own instead of the calendar: a departure, a birth, an ignition. On the timeline's form, turn on Advanced and choose Relative under Date types, then say what the epoch is:

• Anchored to a date keeps the timeline on real dates and adds the epoch on top. Every item can still be written as a calendar date, or as a relative one — Relative appears in the calendar picker once the timeline has an epoch.
• Floating (no real dates) has no calendar at all: the timeline's own start and end, and every item on it, are written relative to the epoch. Such a timeline has to end somewhere, so Ongoing is not offered.

A relative date is a count from the epoch — years, months, weeks, days, hours, minutes or seconds, before or after it — and Counting chooses how it reads: Elapsed as an offset (+2 d, +12 y), or Ordinal as the unit that contains it (Day 3, Week 4, so the epoch itself falls in Day 1). Give the epoch a name, such as Departure or Ignition, and that name stands in for it wherever a date counts from it, and marks the moment itself.

Changing the epoch moves every relative date with it. If that pushes items outside the timeline's start and end, or events outside their period, you are asked before anything is deleted or moved, and an epoch can't be removed while any date still counts from it.

Custom calendars

A timeline with no real dates can also have a custom calendar, for a world you made up: "3 Frostfall 3019" or "T.A. 3019" instead of "Day 1,094". Such a timeline still has to end somewhere, so Ongoing is not offered. A calendar is kept in your account or a team, and many timelines can share it. On the timeline's form, turn on Advanced and choose Custom under Date types to pick one, or to make one on a Pro or Team plan, with its own months, days outside the months, leap years and eras. Custom calendars covers setting one up and writing dates in it.

How the viewer shows a date

A date is kept as you wrote it, and that is how it reads everywhere but the viewer, where each reader chooses how the timeline's dates read for them — under Date display in the header, or from the phone menu. The choice is theirs alone and applies to that timeline only. Display calendar shows every date as entered, or converts it into one calendar — Gregorian, Julian, Hijri, Hebrew, Coptic, Ethiopian, French Republican or Persian — or, on a timeline with an epoch, into relative time counted from it. Era style then sets how a Gregorian or Julian year is numbered: Calendar keeps each date in its own words, as it was written; BCE·CE and Astronomical re-spell Buddhist, Holocene and the other regional-era dates, and After present ages, in that numbering; Before·after present turns every date into an age: a bare one before 1950 (66 Ma) and one marked AP after it (4.5 Ga AP). Left on Auto, Timescrolly shows Gregorian dates from the year 1 CE onward, BCE·CE for anything before that, and switches to ages once a date lies more than ten thousand years before 1950 or beyond the year 285,000,000; Julian dates follow the same rules. A floating timeline counts from its epoch; with a custom calendar, readers choose between that calendar and relative time.

Seeing how sure a date is

Zoom in far enough for a date's uncertainty to show (a year-only date, or one entered with ±) and a period bar's edge softens while an event gets small whiskers on either side. How soft isn't drawn; it means "not exact at this zoom". Select an item to see its full range as a shaded band.

Change this, or turn it off, under Date uncertainty in the viewer's Display menu (header button on desktop, phone menu on phones): the marker style, Soft from, and whether ranges show for the selected item, every item or never. Signed in, it's also under Account › Preferences › Timeline viewer, saved to your account; signed out, this browser remembers it.