vdate.ts

type VDate = Date

type VDateContext = {
  current: (actualBase?: Date) => VDate;
  toVirtual: (date: Date) => VDate;
  toActual: (date: VDate) => Date;
  mapToVirtual: (date: Date, actualBase?: Date) => VDate;
}

const createContext = (factor: number, _virtualBase?: Date): VDateContext => {
  const virtualBase = _virtualBase ?? new Date(new Date().getFullYear(), 0, 1)

  if (factor === 1) {
    const clone = (date: Date) => new Date(date.getTime())

    return {
      current: (actualBase) => actualBase ?? new Date(),
      toVirtual: clone,
      toActual: clone,
      mapToVirtual: clone,
    }
  }

  const toVirtual = (d: Date) => new Date(virtualBase.getTime() + (d.getTime() - virtualBase.getTime()) * factor)
  const toActual = (d: Date) => new Date(virtualBase.getTime() + (d.getTime() - virtualBase.getTime()) / factor)

  const mapToVirtual = (d: Date, actualBase?: Date) => {
    const base = actualBase ?? new Date()
    const diff = d.getTime() - base.getTime()
    return new Date(toVirtual(base).getTime() + diff)
  }

  return {
    current: (actualBase) => toVirtual(actualBase ?? new Date()),
    toVirtual,
    toActual,
    mapToVirtual
  }
}

const context = createContext(3600)

// 2020-04-28T00:00:00.000Z
const current = new Date(2020, 3, 28)
console.log('Current:', current.toISOString())

// 2020-04-28T01:00:00.000Z
const date = new Date(current.getTime() + 60 * 60 * 1000)
console.log('Target: ', date.toISOString())

// 3183-01-24T01:00:00.000Z
const virtual = context.mapToVirtual(date, current)
console.log('Virtual:', virtual.toISOString())

// 2020-04-28T00:00:01.000Z
const actual = context.toActual(virtual)
console.log('Actual: ', actual.toISOString())

これ実行は TZ=UTC node ./vdate.ts