function createDate (epoch) {
  // If epoch is already a valid argument, return the valid Date
  let date = new Date(epoch);
  if (isValidDate(date)) {
    return date
  }

  // Convert to a number to permit epoch as a string
  date = new Date(+epoch);
  return date
}
Constructs a JS Date from a number or string. Accepts any single number
or single string argument that is valid for the Date() constructor,
or an epoch as a string.

@param {string|number} epoch The representation of the Date.

@returns {Date} The constructed Date.