Simple program that simulates StackOverflow post
Just keeping a count is fine for the example app, you don't want to go mad with your implementation. Consider that in real life, you need to know who voted and when. There are various rules about when you can change your vote and when it's locked in. So although your code is fine for the simple 'show me the score' question there are some that you can't answer:


How many votes has the post had in the last day?
What is the split of up and down votes?
Am I allowed to change my vote now?
Have I already voted?


All of those questions (and more) would need to be supported by your final design. If you just wanted to support 2 for now, you could track the up and down votes separately.

As for your code, I'd split out the up and down votes and add a get only property called Score.

Worth noting, if you plan on supporting multiple regions you have to store you dates and times unambiguously. Usually, that means storing in UTC.

so 


  DateTime.Now


Should have been

DateTime.UtcNow


If you just used the first, consider what would happen around any Daylight Saving Time transitions. When the clock goes back there is an entire hour of repeated local times. You won't be able to say which instant in UTC that the time corresponded to.

Edit

The minimal change I'd make to your code is:

public class Post
{
    public string Title { get; set; }
    public string Description { get; set; }

    // C#6 syntax for readonly property
    public DateTime CreationDateTime { get; } = DateTime.UtcNow;

    public int UpVoteCount { get; private set; }
    public int DownVoteCount { get; private set; }

    // C#6 expression bodied member
    public int Score =&gt; UpVoteCount - DownVoteCount;

    public void UpVote()
    {
        UpVoteCount++;
    }

    public void DownVote()
    {
        DownVoteCount++;
    }
}