My Server time Zone was +00:00 GMT and today I found the time showing +1:00 GMT . Would you please tell how it has been changed ? Is that because of Daylight Saving issue ?

The server currently is set to British Summer Time, which is UTC +1 currently, you can change the time zone to UTC/GMT which does not adjust for Daylight Saving Time, you can run the following commands to set the time zone to UTC:

mv /etc/localtime /etc/localtime.bak
ln -s /usr/share/zoneinfo/UTC /etc/localtime

You can prevent this from being changed by updates by running:

chattr +i /etc/localtime