Since the "real" Hogwarts is in the United Kingdom, all official HOL times are in GMT (Greenwich Meridian Time) or BST (British Summer Time), depending on the time of year. This is always the time London is in, regardless of daylight saving, etc. To convert this time to where you live, you add or subtract the number of hours' difference you are from it (example: EST (Eastern Standard Time) is GMT -5)
Follow this link if you are unsure: Time Zone Converter
All pages on the main HOL pages have the HOL-Time in the upper right corner. It might be one hour off, mostly in March and October/November when daylight saving time is introduced or finished on different days throughout the world. Any meeting, Quidditch match, etc. will be scheduled according to HOL-Time. Deadlines for projects, homework, or submissions of any kind are always stated in HOL-Time. Log entries are automatically converted to HOL-Time as well.