She will engaged Troywithin thirty days.
 She will get engaged to Troyin thirty days.
Get engaged within 30 days
Meaning: some day between today and 30 days from now--inside of 30 days; e.g., she and Troy will make a pledge tomorrow or in two days or next week, that they will marry each other on some day in the future that has yet to be set.
Get engaged in30 days
Meaning: thirty days from today;e.g., today is the 7th of June, so she and Troy will make a pledge 30 days from today, on July 6th, that they will marry each other on some day in the future that has yet to be set.
Does that help?
OK. That's really clear. In Thirty days there will be ONLY the pledge, the promise. Quite formal, isn't it? Thank You very much for your attention and first of all for your patience.