Project doesn't allow you to define specific start and end dates for a
task - it is not designed to document a schedule you have already created
and it simply will not really allow you to enter them either with a formula
or by typing them. Entering a date in the "Start" column does NOT set the
start date, instead it sets a "Start No Earlier Than" constraint, setting
the earliest date the task may be scheduled to start but other factors
might drive it out to start later. Entering something into the "Finish"
column similarly sets a "Finish No Earlier Than" constraint and does not
actually define an absolute finish date.
When you enter dates in both the start and the finish columns, you only get
one constraint, not two. The one that is established is determined by your
workflow with the last entry you made being to one that takes effect. So
entering a start then a finish results in a FNET constraint but entering the
finish then the start results in a SNET.
Both starts and finishes are calculated values - calculating them for you is
why Project exists in the first place - with the finish being the start +
duration. So if you want your finish date to be 12 months after the start
date, set the duration to 12 months.
BTW, single tasks that run 12 months are very unusual. A "task" is a bundle
of work producing a single deliverable done by a single resource or resource
team. It turns out in the real world that 99% of tasks run between 8 and 80
hours. Under 8 and you're probably excessively micromanaging. Over 80 and
you haven't broken the work down into enough detail to effectively manage
it. There are exceptions, of course, but if you find you have tasks running
outside those limits alarm bells should go off to alert you to take a second
look at your breakdown.
