When they need to write the Gregorian date
Question

                Some of the offices of al-Muntada al-Islami are in countries which use the Gregorian calendar. We use the Hijri calendar, but this causes us problems when we deal with government departments which do not use the Hijri calendar, or when drawing up budgets, paying salaries and preparing financial reports for the appropriate departments in those countries.
Praise be to Allah.There is nothing wrong with using both dates, so long as
    the Hijri date is written first, followed by the equivalent Gregorian date. The Hijri date
    is based on the lunar months, which are clear and can be readily observed by anyone who
    has eyes to see, but the Gregorian months do not have any clearly visible signs and can
    only be known through calculations. Hence the Islamic sharee’ah uses the Arabic
    months for the dates of fasting, Hajj, I’tikaaf and so on. Beginning with the Hijri
    date is also a manifestation of the symbols and features of Islam among those who do not
    know them. And Allaah knows best.