adjustDate

index.ts

import {
  subHours,
  subMinutes,
} from "date-fns"
export const DEFAULT_CUSTOM_DAY_START = "00:00:00"
declare const _dateAdjusted: unique symbol
export type DateAdjustedToCustomDayStart = Date & {
  readonly [_dateAdjusted]: Date
}

/**
 * Helper function to adjust the date based on the customDayStart.
 */
export function adjustDate(
  date: Date,
  customDayStart: string | null
): DateAdjustedToCustomDayStart {
  if (!customDayStart || customDayStart === DEFAULT_CUSTOM_DAY_START) {
    return date as DateAdjustedToCustomDayStart
  }
  // Split up the customDayStart into hours/minutes
  // so we can subtract them
  const [hours, minutes] = customDayStart
    .split(":", 3)
    .map((n) => parseInt(n, 10))

  return subHours(
    subMinutes(date, minutes),
    hours
  ) as DateAdjustedToCustomDayStart
}