Dought & clear, - When they need to write the Gregorian date.

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 Allaah.
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.