Except in very rare special circumstances you should never 'specify' a
task's start date/time and/or its end date/time. What you do input is the
task's duration and what you describe is a 30 minute duration task, entered
in the duration column as '30 min'. When you link it to its predecessor and
assign the resource who will do work, Project will determine for you, based
on when the predecessor is done and when the resource's calendar says he is
available, when the task can start and will also calculate from the start
plus the duration when it will end.
