Firebase PushID generator in C# (ref: https://firebase.googleblog.com/2015/02/the-2120-ways-to-ensure-unique_68.html) JavaScript gist https://gist.github.com/mikelehen/3596a30bd69384624c11

FirebasePushIDGenerator.cs

void Main()
{
    var pushId = FirebasePushIDGenerator.GeneratePushID();
    Console.WriteLine(pushId);
    Console.WriteLine(FirebasePushIDGenerator.GeneratePushID());
    Console.WriteLine(FirebasePushIDGenerator.GeneratePushID());
    Thread.Sleep(5);
    Console.WriteLine(FirebasePushIDGenerator.GeneratePushID());
    Thread.Sleep(5);
    Console.WriteLine(FirebasePushIDGenerator.GeneratePushID());
    Thread.Sleep(5);
    Console.WriteLine(FirebasePushIDGenerator.GeneratePushID());
    
    Console.WriteLine("Convert '{0}' to unix time ms", pushId);
    Console.WriteLine(FirebasePushIDGenerator.ConvertPushID(pushId));
    
}


public class FirebasePushIDGenerator
{
    /**
     * Fancy ID generator that creates 20-character string identifiers with the following properties:
     *
     * 1. They're based on timestamp so that they sort *after* any existing ids.
     * 2. They contain 72-bits of random data after the timestamp so that IDs won't collide with other clients' IDs.
     * 3. They sort *lexicographically* (so the timestamp is converted to characters that will sort properly).
     * 4. They're monotonically increasing.  Even if you generate more than one in the same timestamp, the
     *    latter ones will sort after the former ones.  We do this by using the previous random bits
     *    but "incrementing" them by 1 (only in the case of a timestamp collision).
     */
    // Modeled after base64 web-safe chars, but ordered by ASCII.
    const string PUSH_CHARS = "-0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyz";

    // Timestamp of last push, used to prevent local collisions if you push twice in one ms.
    private static long lastPushTime = 0;

    // We generate 72-bits of randomness which get turned into 12 characters and appended to the
    // timestamp to prevent collisions with other clients.  We store the last characters we
    // generated because in the event of a collision, we'll use those same characters except
    // "incremented" by one.
    private static char[] lastRandChars = new char[12];

    // Random number generator
    private static Random rng = new Random();
    
    public static string GeneratePushID()
    {
        var now = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
        Console.WriteLine("now=" + now);

        var duplicateTime = (now == lastPushTime);
        lastPushTime = now;

        var timeStampChars = new char[8];
        for (var i = 7; i >= 0; i--)
        {
            timeStampChars[i] = PUSH_CHARS[(int)(now % 64)];
            now = now >> 6;
        }
        if (now != 0) throw new Exception("We should have converted the entire timestamp.");

        var id = string.Join(string.Empty, timeStampChars);

        if (!duplicateTime)
        {
            for (var i = 0; i < 12; i++)
            {
                lastRandChars[i] = (char)rng.Next(0, 63);
            }
        }
        else
        {
            // If the timestamp hasn't changed since last push, use the same random number, except incremented by 1.
            int i;
            for (i = 11; i >= 0 && lastRandChars[i] == 63; i--)
            {
                lastRandChars[i] = (char)0;
            }
            lastRandChars[i]++;        
        }
        for (var i = 0; i < 12; i++)
        {
            id += PUSH_CHARS[lastRandChars[i]];
        }
        if (id.Length != 20) throw new Exception("Length should be 20.");

        return id;
    }

    public static long ConvertPushID(string id)
    {
        var timestamp = 0L;
        for (var i = 0; i < 8; i++)
        {
            var n = PUSH_CHARS.IndexOf(id[i]);
            timestamp = (timestamp << 6) + n;
        }
        return timestamp;
    }
}

gistfile2.txt

Output. Note the first 3 timestamps had identical milliseconds, so last digit of random bits was incremented by 1.
Also converts first PushID back in to unix timestamp. Verify that they are the same.

now=1476984292596
-KUY6wIoxwYFJbKhYuxA

now=1476984292596
-KUY6wIoxwYFJbKhYuxB

now=1476984292596
-KUY6wIoxwYFJbKhYuxC

now=1476984292602
-KUY6wIugap8n5ZN1UPx

now=1476984292608
-KUY6wJ-uMBurx137YK7

now=1476984292613
-KUY6wJ4amFnKQlcEgiJ

Convert '-KUY6wIoxwYFJbKhYuxA' to unix time ms
1476984292596