Hijri Date Today shows today's Islamic date for 14 countries. Countries do not always agree, so each one gets its own row: Saudi Arabia on Umm al-Qura, others on their local moon-sighting decision. A comparison table shows where they differ. If your mosque is a day off, use the ±1 day adjust.
There is also a converter that works both ways. You can paste a list of dates and export the result as CSV. The conversion runs in your browser. A full 1448 AH calendar is included.