Tamil

I have been working on a website where users from all around the world would post their suggestions, upload photos and create events for hangouts and so on. Its just like a social network. I need to how to store the Data and time of all the users in the correct format so that it would be the best choice for the future of the website. For example if a user in US creates an event then a person in india seeing it must be seeing it correctly in his time format. If he chooses to fly to US and login's in US then the time for the user in that specific timezone must match. The thing is any where the user may be, any time or date related activity in the website must store the time which can be common to all. I am aware of the two things, UTC and GMT where both are similary but with the DST. I dono which one to use wether to use GMT or UTC. Should i calculate time by adding or subtracting the offset, or coverting the timezone stored to the timezone of the user. I also need to know how to get the timezone or the offset of the user for a specific region. Thanks.