Steps to reproduce:

1. Create a new console app
2. Edit Program.cs to print it's last write time out:

using System;
using System.IO;

namespace ts_test
{
    class Program
    {
        static void Main(string[] args)
        {
            var file = Path.Combine(Directory.GetCurrentDirectory(), "Program.cs");
            var t = File.GetLastWriteTimeUtc(file);

            Console.WriteLine(t.ToString("o"));
        }
    }
}

On my Windows machine, here's the output:

2017-12-22T04:59:47.5871375Z

On my Ubuntu VM, here's the output:

2017-12-22T05:18:38.0000000Z

Notice the missing precision after the seconds digits. The filesystem does seem to have a higher precision timestamp associated with the file:

>:~/src/ts-test$ ls --full-time Program.cs
-rw-rw-r-- 1 prkrishn prkrishn 320 2017-12-22 05:18:38.250041900 +0000 Program.cs

 dotnet --info
.NET Command Line Tools (2.0.2)

Product Information:
 Version:            2.0.2
 Commit SHA-1 hash:  a04b4bf512

Runtime Environment:
 OS Name:     ubuntu
 OS Version:  16.04
 OS Platform: Linux
 RID:         ubuntu.16.04-x64
 Base Path:   /usr/share/dotnet/sdk/2.0.2/

Microsoft .NET Core Shared Framework Host

  Version  : 2.0.0
  Build    : e8b8861ac7faf042c87a5c2f9f2d04c98b69f28d

Further background:

I noticed this when I was writing tests to cover incremental build scenarios in my MSBuild target. MSBuild uses file timestamps to determine if the inputs are consistent with the outputs. Consequently inputs modified within a second of creating the output would be treated as consistent resulting in MSBuild target executions from being skipped. I also some my tests fail on Travis OSX 10.12 but I don't have a machine handy to confirm if this was the cause.