Are you worried about observing your favorite historical anniversaries on the correct date? The internet has the answer.
Let's say that you're obsessed with the Roman emperor Valens and want to celebrate or mourn his catastrophic defeat and death at the Battle of Adrianople on August 9, 378 AD. But wait! Something's bugging you. August 9, 378 was the date of the battle according to the calendar of the time - the Julian Calendar. To properly observe the event, you realize, you need to figure out what that date translates to on the Gregorian Calendar we now use.
It turns out there are multiple calendar converters on the web that will accomplish the task in just a few seconds. Here's one of many that I found, which appears no better or worse than many others. It turns out that Thursday August 9, 378 (Julian Calendar) would have been August 10 on the Gregorian Calendar.