19 tools
Converters & validators
Julian days, Excel serials, GPS time, cron expressions and format validators. Each tool has its own page. It runs in your browser, and it keeps its inputs in the address bar so you can share the answer.
The tools
Convert Julian days, Excel and SQL serial dates, Oracle dates and GPS time. Build and explain cron expressions, check date strings, pick random dates.
Julian Date ConverterThe astronomer's continuous day count, both ways — including the half-day offset that catches everyone, since a Julian day begins at noon rather than midnight.
Julian Day Number CalculatorThe whole-number form — the JD of the noon that starts a day — with the Lilian and Rata Die day counts beside it, which are the same idea from different zero points.
Modified Julian Date ConverterMJD — the Julian date with 2,400,000.5 taken off, so it starts at midnight and fits in five digits. Used in satellite tracking, geodesy and time metrology.
GPS Time ConverterGPS seconds, week number and time-of-week, converted with the leap-second offset applied — because GPS time has taken none since 1980 and is now 18 seconds ahead of UTC.
Excel Date ConverterA spreadsheet serial to a calendar date and back, in both the 1900 and 1904 systems — with the phantom leap day of 1900 flagged where it matters.
Oracle Date ConverterThe seven-byte internal form, decoded — century and year stored excess-100, everything else plus one — plus the Julian day that `TO_CHAR(d,'J')` gives you.
SQL Date FormatterOne instant, rendered as a literal for PostgreSQL, MySQL, SQL Server, Oracle, SQLite, BigQuery and Snowflake — plus how each spells 'now' and how each reads a Unix timestamp.
ISO 8601 ValidatorA strict check against the standard, with the specific rule any failure broke — and a note on whether it would also pass as RFC 3339, which is stricter still.
Time String ValidatorChecks a time of day against the standard — including the edge cases that are legal and surprising, like 24:00 and the leap-second 23:59:60.
Date ValidatorChecks that a date exists as well as parses — 31 February and 29 February in a common year both parse fine in most languages, and neither is a date.
Timestamp ValidatorChecks a number timestamp for the mistakes that really happen. The unit may be wrong. It may be past the 32-bit limit. It may be a stand-in value. Or the date may be far from where it should be.
Date RandomizerOne random date between two bounds, with a seed so the same seed always gives the same date — for anything you need to be able to reproduce.
Random Date GeneratorAs many random dates as you need, in a range, sorted or shuffled, with the format you want — CSV, JSON, SQL literals or Unix timestamps.
Random Time GeneratorRandom times within a window — working hours, overnight, anything — one at a time or in bulk, with a seed for repeatability.
Cron Expression GeneratorPick a schedule and get the expression, described back to you in English and with its next five runs shown — so you can see what it does before it does it.
Cron Expression ParserPaste an expression and get it back in English, field by field, with its next runs, how often it fires, and any trap it has walked into.
Cron Expression ValidatorChecks the syntax, names the field that failed, and warns about the expressions that are valid but never fire — 30 February being the classic.
Cron Human Readable ConverterOne sentence describing what an expression does, plus a field-by-field breakdown — for putting in a comment, a runbook or a pull request description.
Relative Time GeneratorThe phrasing a feed or a comment thread uses, in any locale, with the thresholds shown — and a note on when relative time is the wrong choice.