Bash, 17 16 15 14 13 bytes

date -d100day

It turns out the date command takes some pretty flexible input for relative timings. You can also do things like 1 year, 1 week ago, yesterday, etc. It's pretty cool.

-1 byte by realizing that bash does not care about grammar.
-1 byte because the space between 100 and day is unnecessary.
-1 byte because I don't need quotes anymore because I don't have a space in the string.
-1 byte by removing the space after -d (thanks ASCII-only!)

Excel, 10

=NOW()+100

(Body must be at least 30 characters)

JavaScript, 42 bytes

(d=new Date()).setDate(d.getDate()+100)&&d

setInterval(_=>document.body.innerHTML=(d=new Date()).setDate(d.getDate()+100)&&d,1000)

Python, 63 bytes

from datetime import*
print(datetime.now()+timedelta(days=100))

Very simple solution really. Because the datetime.now() result is the same type as the result of timedelta, it happily adds the two together.

C#, 103 bytes

using System;class P{static void Main(){Console.Write(DateTime.Now.AddDays(100).ToShortDateString());}}

Full program which reads the current date, adds 100 days and displays the result in M/d/YYYY format.

You can change the date format by adding a few more bytes (one byte in the following):

using System;class P{static void Main(){Console.Write(DateTime.Now.AddDays(100).ToString("d-M-yyyy"));}}

To eliminate boilerplate code - C# isn't exactly known to be very compact - an anonymous function can be used:

C# lambda, 49 bytes

()=>DateTime.Now.AddDays(100).ToShortDateString()

Full program:

using System;

class P
{
    static void Main()
    {
        Func<string> f = 
        ()=>DateTime.Now.AddDays(100).ToShortDateString();

        Console.WriteLine(f());
    }
}

Mathematica, 26 bytes

Today+Quantity[100,"Days"]

If including the time is permitted, then we can save 2 bytes:

Now+Quantity[100,"Days"]

PHP, 28 bytes

<?=date(Y_m_d,time()+864e4);

human readable version, 35 bytes:

<?=date(Y_m_d,strtotime("100day"));

almost readable, 34 bytes:

<?=date(Y_m_d,strtotime(1e2.day));

-4 bytes if also printing the time is accepted: replace Y_m_d with r or c.

It´s National Holiday in France in 100 days.

IBM/Lotus Notes Formula, 29 bytes

@Adjust(@Today;0;0;100;0;0;0)

Unfortunately @Adjust requires the trailing 0's for the missing hh:mm:ss.

If we were allowed to display the time as well then changing @Today to @Now would save 2 bytes for 27.

Ruby, 16 bytes

p Time.now+864e4

Try it online!

JavaScript, 38 Bytes

d=new Date;d.setDate(d.getDate()+100);d

A little bit shorter than ASCII-only answer.

document.write(eval("d=new Date;d.setDate(d.getDate()+100);d"))