Use this calendar calculation when dates follow the stated calendar and you need a reproducible boundary, interval, ordinal, week, or classification result.

Date

Required string input.

Return the first day of the month of an ISO calendar date. The tool evaluates the supplied inputs together and returns the named outputs below; it does not infer omitted operating conditions or change the units shown.^[1]

Start of month

The resulting start of month returned as a string.

• Calendar dates are not elapsed-time instants by themselves. Time zones, daylight-saving transitions, locale-specific week rules, inclusive endpoints, holidays, historical calendar reforms, and proleptic-calendar assumptions can change an apparently simple result.
• Use finite inputs in the displayed units and preserve more precision than the final presentation requires. Independently verify safety-critical, financial, compliance, or production decisions.

Store instants with an explicit time zone when time of day matters, and document whether interval endpoints are inclusive. Supply a jurisdiction-specific holiday calendar for business-day decisions.

• End of Month Tool

  Return the last day of the month of an ISO calendar date.

• Fiscal Quarter Calculator

  Return the fiscal quarter (1-4) of an ISO calendar date for a fiscal year that starts in a chosen month.

• Zodiac Sign Calculator

  Return the western zodiac sign for a calendar month and day using standard date ranges.