/**
 * Normalize a date-time unit string to its singular form.
 *
 * - Accepts singular ("day"), plural ("days"), and already-singular forms.
 * - Returns the original string if it is not a recognized DateTimeUnit.
 * - Singular counterpart of `resolveDurationUnit`, which normalizes to plurals.
 *
 * @param unit raw unit string from caller input
 * @returns normalized singular unit string, or the original if not recognized
 */
export declare function resolveDateTimeUnit(unit: string): string;