Okay, we can fix that.   Take it out of the control source and move it to a
private function.  At the very top of your form module (the code attached to
the form), and just after all the Option statements, create this sub:

Private Sub CalcEndDate()
   If Not IsNull(Me.[start_date]) And Not IsNull(Me.[length]
       Me.txtProjectedEndDate = DateAdd("m",Me.[length],Me.[start_date])
   End If
End Sub

Now, use the after update event for lenght and start_date to call the
function:

Private Sub start_date_AfterUpdate()
   Call CalcEndDate
End Sub

Since you don't know in what order the user will enter data or whether they
will visit any control at all, the Sub above will only calculate the value
when both controls have a value.

A word about naming, before you go any further:
I am assuming the [length] and [start_date] are the names of controls on
your form.  They may also be the names of fields in your table if you created
the form using a wizard.  The wizard has a very bad habit.  It names the
controls the same name as the field it is bound to.  This can cause a lot of
confusion.  They really should not be the same name.

User defined names should only contain letters, numbers, and the underscore.
Never use spaces or any special character other than the underscore in names.
Never use any Access reserved word (Date, Month, Name, etc.).  They can
confuse Access.  If you have a name that does not conform to the above rules,
enclose it in brackets to avoid problems.  [Date]

Here is a link to a site that will give you some info on good naming
conventions:

http://www.mvps.org/access/general/gen0012.htm

Using standard naming conventions makes your life easier and is doing a big
favor to the person who has to come behind you to support your code when you
win the Lottery :)

Good work, We are making progress.