export declare function startOfDay(date: Date): Date;
export declare function isSameDay(a: Date, b: Date): boolean;
export declare function isSameMonth(a: Date, b: Date): boolean;
export declare function isToday(date: Date): boolean;
/** DST-safe: always adds calendar days, not 86400-second units. */
export declare function addDays(date: Date, days: number): Date;
/** DST-safe: changes month on same calendar day, clamping to end of month. */
export declare function addMonths(date: Date, months: number): Date;
export declare function addYears(date: Date, years: number): Date;
export declare function startOfMonth(date: Date): Date;
export declare function endOfMonth(date: Date): Date;
export declare function daysInMonth(year: number, month: number): number;
export declare function isLeapYear(year: number): boolean;
/**
 * ISO 8601 week number.
 * Week 1 = the week that contains the first Thursday of the year.
 */
export declare function isoWeekNumber(date: Date): number;
/**
 * US week number: week 1 starts on the first Sunday of the year (or Jan 1).
 */
export declare function usWeekNumber(date: Date): number;
export declare function weekNumber(date: Date, system?: 'iso' | 'us'): number;
export declare function isWeekend(date: Date): boolean;
/** Normalise a weekday (0=Sun) to an offset from the given firstDay (0=Sun,1=Mon,...). */
export declare function weekdayOffset(day: number, firstDay: number): number;