react-native-date-ranges-picker/src/dates.js

This is a fork of react-native-date-ranges a bit more customizable

export const dates = (startDate, endDate, focusedInput ) => {
  if (focusedInput === 'startDate') {
    if (startDate && endDate) {
      return ({ startDate, endDate: null, focusedInput: 'endDate' });
    }
    return ({ startDate, endDate, focusedInput: 'endDate' });
  }

  if (focusedInput === 'endDate') {
    if (endDate && startDate && endDate.isBefore(startDate)) {
      return ({ startDate: endDate, endDate: null, focusedInput: 'endDate' });
    }
    return ({ startDate, endDate, focusedInput: 'startDate' });
  }

  return ({ startDate, endDate, focusedInput });
};