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+import toDate from '../toDate/index'
+import isValid from '../isValid/index'
+import addLeadingZeros from '../_lib/addLeadingZeros/index'
+
+/**
+ * @name formatISO
+ * @category Common Helpers
+ * @summary Format the date according to the ISO 8601 standard (http://support.sas.com/documentation/cdl/en/lrdict/64316/HTML/default/viewer.htm#a003169814.htm).
+ *
+ * @description
+ * Return the formatted date string in ISO 8601 format. Options may be passed to control the parts and notations of the date.
+ *
+ * @param {Date|Number} date - the original date
+ * @param {Object} [options] - an object with options.
+ * @param {'extended'|'basic'} [options.format='extended'] - if 'basic', hide delimiters between date and time values.
+ * @param {'complete'|'date'|'time'} [options.representation='complete'] - format date, time with time zone, or both.
+ * @returns {String} the formatted date string
+ * @throws {TypeError} 1 argument required
+ * @throws {RangeError} `date` must not be Invalid Date
+ * @throws {RangeError} `options.format` must be 'extended' or 'basic'
+ * @throws {RangeError} `options.represenation` must be 'date', 'time' or 'complete'
+ *
+ * @example
+ * // Represent 18 September 2019 in ISO 8601 format (UTC):
+ * const result = formatISO(new Date(2019, 8, 18, 19, 0, 52))
+ * //=> '2019-09-18T19:00:52Z'
+ *
+ * @example
+ * // Represent 18 September 2019 in ISO 8601, short format (UTC):
+ * const result = formatISO(new Date(2019, 8, 18, 19, 0, 52), { format: 'basic' })
+ * //=> '20190918T190052'
+ *
+ * @example
+ * // Represent 18 September 2019 in ISO 8601 format, date only:
+ * const result = formatISO(new Date(2019, 8, 18, 19, 0, 52), { representation: 'date' })
+ * //=> '2019-09-18'
+ *
+ * @example
+ * // Represent 18 September 2019 in ISO 8601 format, time only (UTC):
+ * const result = formatISO(new Date(2019, 8, 18, 19, 0, 52), { representation: 'time' })
+ * //=> '19:00:52Z'
+ */
+export default function formatISO(dirtyDate, dirtyOptions) {
+  if (arguments.length < 1) {
+    throw new TypeError(
+      `1 argument required, but only ${arguments.length} present`
+    )
+  }
+
+  const originalDate = toDate(dirtyDate)
+
+  if (!isValid(originalDate)) {
+    throw new RangeError('Invalid time value')
+  }
+
+  const options = dirtyOptions || {}
+  const format = options.format == null ? 'extended' : String(options.format)
+  const representation =
+    options.representation == null ? 'complete' : String(options.representation)
+
+  if (format !== 'extended' && format !== 'basic') {
+    throw new RangeError("format must be 'extended' or 'basic'")
+  }
+
+  if (
+    representation !== 'date' &&
+    representation !== 'time' &&
+    representation !== 'complete'
+  ) {
+    throw new RangeError("representation must be 'date', 'time', or 'complete'")
+  }
+
+  let result = ''
+  let tzOffset = ''
+
+  const dateDelimiter = format === 'extended' ? '-' : ''
+  const timeDelimiter = format === 'extended' ? ':' : ''
+
+  // Representation is either 'date' or 'complete'
+  if (representation !== 'time') {
+    const day = addLeadingZeros(originalDate.getDate(), 2)
+    const month = addLeadingZeros(originalDate.getMonth() + 1, 2)
+    const year = addLeadingZeros(originalDate.getFullYear(), 4)
+
+    // yyyyMMdd or yyyy-MM-dd.
+    result = `${year}${dateDelimiter}${month}${dateDelimiter}${day}`
+  }
+
+  // Representation is either 'time' or 'complete'
+  if (representation !== 'date') {
+    // Add the timezone.
+    const offset = originalDate.getTimezoneOffset()
+
+    if (offset !== 0) {
+      const absoluteOffset = Math.abs(offset)
+      const hourOffset = addLeadingZeros(Math.floor(absoluteOffset / 60), 2)
+      const minuteOffset = addLeadingZeros(absoluteOffset % 60, 2)
+      // If less than 0, the sign is +, because it is ahead of time.
+      const sign = offset < 0 ? '+' : '-'
+
+      tzOffset = `${sign}${hourOffset}:${minuteOffset}`
+    } else {
+      tzOffset = 'Z'
+    }
+
+    const hour = addLeadingZeros(originalDate.getHours(), 2)
+    const minute = addLeadingZeros(originalDate.getMinutes(), 2)
+    const second = addLeadingZeros(originalDate.getSeconds(), 2)
+
+    // If there's also date, separate it with time with 'T'
+    const separator = result === '' ? '' : 'T'
+
+    // Creates a time string consisting of hour, minute, and second, separated by delimiters, if defined.
+    const time = [hour, minute, second].join(timeDelimiter)
+
+    // HHmmss or HH:mm:ss.
+    result = `${result}${separator}${time}${tzOffset}`
+  }
+
+  return result
+}