Question

I am capturing some events in a C# program that come back with the timestamp as a system tickcount (milliseconds since start time).

Knowing, based on other questions I’ve seen, that I can get the same number from System.Environment.TickCount property (or something else), how can I infer the DateTime object that corresponds to the TickCount I received?

Answer 1

You can’t, without more information (and even then it can be ambiguous). Environment.TickCount returns:

A 32-bit signed integer containing the amount of time in milliseconds that has passed since the last time the computer was started.

… so unless you can find out the time the computer was started from somewhere, you’re out of luck. There may well be registry entries or system calls you can make to find out the last boot time, but I don’t know them off the top of my head. Of course, you can get an approximate value by taking Environment.TickCount yourself and DateTime.UtcNow as soon after (or before) that as possible, and finding the difference between the two:

public static DateTime UnreliableDateTimeFromTickCount(int tickCount)
{
    DateTime now = DateTime.UtcNow;
    DateTime boot = now - TimeSpan.FromMilliseconds(Environment.TickCount);
    return boot + TimeSpan.FromMilliseconds(tickCount);
}

However, even with that, the value will cycle round every 24.9 days, so if the computer has been on for longer than that, the count is ambiguous.

I’d suggest avoiding using Environment.TickCount if possible, basically – is this under your control at all?