/**
 * Take the date-part from `newDate` and the time-of-day from `prev`. When
 * `prev` is null/invalid the merged time-of-day defaults to midnight.
 */
export declare function mergeDate(prev: Date | null, newDate: Date): Date;
/**
 * Take the time-of-day from `newTime` and the date-part from `prev`. When
 * `prev` is null/invalid the merged date-part defaults to today.
 */
export declare function mergeTime(prev: Date | null, newTime: Date): Date;
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