Develop Reference

Why is time not incrementing

I am trying to show the timestamp of when a process begins and when it completes, but my time never increments (even though time in reality does)
DateTime now = DateTime.Now;
txt1.AppendText(now + Environment.NewLine);
//Lengthy process that usually takes 2 - 3 minutes
txt1.AppendText(now);

You are literally using the same variable with the same value as when you first displayed it. You would need to get the time again like this...
DateTime now = DateTime.Now;
txt1.AppendText(now + Environment.NewLine);
//Lengthy process that usually takes 2 - 3 minutes
DateTime timeHasPassed = DateTime.Now;
txt1.AppendText(timeHasPassed);

DateTime is a ValueType. When you assign it to a variable, you are making a copy of the value, not the reference.
This means that when you use DateTime.Now, you have to invoke it via the Get Property to get the latest time value.
txt1.AppendText(DateTime.Now + Environment.NewLine);
//Lengthy process that usually takes 2 - 3 minutes
txt1.AppendText(DateTime.Now);

That's because you captured and stored the DateTime.Now in a variable so it shows the stored value. Use DateTime.Now again instead of using now variable.

Don't use DateTime to measure your process time of your code.
You should never do any increment or math with DateTime.Now because it may be ambiguous due to DST and TimeZone issues.
Use StopWatch to measure elapsed time instead. This class offers high-precision timing in .NET. It is capable of measuring time with sensitivity of around 100s of nanoseconds.
You can use it's Start and Stop methods to control of a StopWatch object.

What is the default time precision of DateTime?

I did a small winform program for data transferring in Visual Studio, and I used a method to provide the transferring time duration. After the transferring being done, the program will return a dialog window to show the time.
But here I don't know what is the time precision or the resolution of the timer, how can it be such a precision, even micro second?
var startTime = DateTime.Now;
this.transferdata();
var endTime = DateTime.Now;
var timeElapsed = endTime.Subtract(startTime);
when I saw the definition of class DateTime, there is only a precision in milisecond. can anybody tell me why there is such a high resolution timer in the visual studio 2012? Or there is related to the operating system?

The precision of the clock depends on the operating system. The system clock ticks a certain number of times per second, and you can only measure whole ticks.
You can test the resolution for a specific computer using code like this:
DateTime t1 = DateTime.Now;
DateTime t2;
while ((t2 = DateTime.Now) == t1) ;
Console.WriteLine(t2 - t1);
On my computer the result is 00:00:00.0156253, which means that the system clock ticks 64 times per second.
(Note: The DateTime type also has ticks, but that is not the same as the system clock ticks. A DateTime tick is 1/10000000 second.)
To measure time more precisely, you should use the Stopwatch class. Its resolution also depends on the system, but is much higher than the system clock. You can get the resolution from the Stopwatch.Frequency property, and on my computer it returns 2143566 which is a tad more than 64...
Start a stopwatch before the work and stop it after, then get the elapsed time:
Stopwatch time = Stopwatch.StartNew();
this.transferdata();
time.Stop();
TimeSpan timeElapsed = time.Elapsed;
That will return the time in the resolution that the TimeSpan type can handle, e.g. 1/10000000 second. You can also calculate the time from the number of ticks:
double timeElapsed = (double)s.ElapsedTicks / (double)Stopwatch.Frequency;

You are confusing several things. Precision, Accuracy, Frequency, and Resolution.
You might have a variable that is accurate to a billion decimal places. But if you can't actually measure that small of a number then that's the difference between precision and resolution. Frequency is the number of times per second a measurement is taken, while relates to resolution. Accuracy is how closely a given sample is to the real measurement.
So, given that DateTime has a precision much higher than the system clock, simply saying DateTime.Now will not necessarily give you an exact timestamp. There are, however, Higher resolution timers in Windows, and the Stopwatch class uses them to measure time elapsed, so if you use this class you get a much better accuracy.
DateTime has no "default precision". It has only one precision, and that's the Minimum and Maximum values it can store. DateTime internally stores it's values as a single value, and this value is formatted to whatever type you want to display (seconds, minutes, days, ticks, whatever...).

datetime checking for specific time

I have a windows service and I would like to insert a timer. How can I check if the present time is 9:00 AM ?
I would like my service to check this every day. Thank you a lot
My try:
Datetime dt=Datetime.parse("09:00:00 everyday");
if(datetime.now -dt ==0)
{
//fire event
}
Thats kinda sily of me though.

You need to make a timer and sets its interval to the timespan between now and tomorrow 9:00 AM. Next time the timer tick, set the interval again in the same way.
You should use this Timer class:
http://msdn.microsoft.com/en-us/library/system.timers.timer.aspx

Please use DateTime.UtcNow.Hour to check current hour
By using UtcNow you will gets a DateTime object that is set to the current date and time on the computer, expressed as the Coordinated Universal Time (UTC).

var now = DateTime.Now;
var today = now.Date;
var nineAm = today.AddHours(9);
TimeSpan ts = nineAm - now;
var timeInMillisecondsTill9Am = ts.Milliseconds;
If(timeInMillisecondsTill9Am==0)
{
//your code goes here
}

Since you don't know when someone may shutdown or reboot your computer or service then you need to make sure that you use a method robust enough to handle these kinds of interruptions.
I suggest that when your service checks every 5 minutes or so if the time is after 9am and if the last run date is yesterday. If so, you update the last run date to day (perhaps in a simple text file) and then run the "9:00am" task. In this way your task only runs once per day, fairly close to 9am, and is robust against reboots.
You'll need to use a standard .NET timer to trigger the checks - and if you're clever enough you can make it fire a few seconds after 9am.
Let me know if that's a good solution.

Get time in milliseconds using C#

I'm making a program in which I need to get the time in milliseconds. By time, I mean a number that is never equal to itself, and is always 1000 numbers bigger than it was a second ago. I've tried converting DateTime.Now to a TimeSpan and getting the TotalMilliseconds from that... but I've heard it isn't perfectly accurate.
Is there an easier way to do this?

long milliseconds = DateTime.Now.Ticks / TimeSpan.TicksPerMillisecond;
This is actually how the various Unix conversion methods are implemented in the DateTimeOffset class (.NET Framework 4.6+, .NET Standard 1.3+):
long milliseconds = DateTimeOffset.Now.ToUnixTimeMilliseconds();

Use the Stopwatch class.
Provides a set of methods and
properties that you can use to
accurately measure elapsed time.
There is some good info on implementing it here:
Performance Tests: Precise Run Time Measurements with System.Diagnostics.Stopwatch

The DateTime.Ticks property gets the number of ticks that represent the date and time.
10,000 Ticks is a millisecond (10,000,000 ticks per second).

I use the following class. I found it on the Internet once, postulated to be the best NOW().
/// <summary>Class to get current timestamp with enough precision</summary>
static class CurrentMillis
{
private static readonly DateTime Jan1St1970 = new DateTime (1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
/// <summary>Get extra long current timestamp</summary>
public static long Millis { get { return (long)((DateTime.UtcNow - Jan1St1970).TotalMilliseconds); } }
}
Source unknown.

You can try the QueryPerformanceCounter native method. See http://www.pinvoke.net/default.aspx/kernel32/QueryPerformanceCounter.html for more information. This is what the Stopwatch class uses.
See How to get timestamp of tick precision in .NET / C#? for more information.
Stopwatch.GetTimestamp() gives access to this method:
public static long GetTimestamp() {
if(IsHighResolution) {
long timestamp = 0;
SafeNativeMethods.QueryPerformanceCounter(out timestamp);
return timestamp;
}
else {
return DateTime.UtcNow.Ticks;
}
}

I used DateTime.Now.TimeOfDay.TotalMilliseconds (for current day), hope it helps you out as well.

Use System.DateTime.Now.ToUniversalTime(). That puts your reading in a known reference-based millisecond format that totally eliminates day change, etc.

Using Stopwatch class we can achieve it from System.Diagnostics.
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
stopwatch.Stop();
Debug.WriteLine(stopwatch.ElapsedMilliseconds);

System.DateTimeOffset.Now.ToString("MM/dd/yyyy hh:mm:ss.fff tt") to get the millisecond in the format of '04/01/2021 04:32:14.788 PM'
https://learn.microsoft.com/en-us/dotnet/standard/base-types/how-to-display-milliseconds-in-date-and-time-values

As I understand your requirements Environment.TickCount fits the bill. It returns number of milliseconds since startup, so it always increases and can be used for computing elapsed time in milliseconds. If you want absolute time also, you can get current time and current Environment.TickCount, and compute new absolute time based on that and new Environment.TickCount.

inaccurate .NET timer?

I'm developing an application and I need to get the current date from a server (it differs from the machine's date).
I receive the date from the server and with a simple Split I create a new DateTime:
globalVars.fec = new DateTime(DateTime.Now.Year, DateTime.Now.Month, DateTime.Now.Day, int.Parse(infoHour[0]), int.Parse(infoHour[1]), int.Parse(infoHour[2]));
globalVars is a class and fec is a public static variable so that I can access it anywhere in the application (bad coding I know...).
Now I need to have a timer checking if that date is equal to some dates I have stored in a List and if it is equal I just call a function.
List<DateTime> fechas = new List<DateTime>();
Before having to obtain the date from a server I was using computer's date, so to check if the dates matched I was using this:
private void timerDatesMatch_Tick(object sender, EventArgs e)
{
DateTime tick = DateTime.Now;
foreach (DateTime dt in fechas)
{
if (dt == tick)
{
//blahblah
}
}
}
Now I have the date from the server so DateTime.Now can't be used here. Instead I have created a new timer with Interval=1000 and on tick I'm adding 1 second to globalVars.fec using:
globalVars.fec = globalVars.fec.AddSeconds(1);
But the clock isn't accurate and every 30 mins the clock loses about 30 seconds.
Is there another way of doing what I'm trying to do? I've thought about using threading.timer instead but I need to have access to other threads and non-static functions.

Store the difference between the server's time and local time. Calculate the servers' time when you need it using that difference.

If you create atimer with an interval of 1000ms, it will be called no sooner than 1000ms. So you can pretty much guarantee that it will be called in more than 1000ms, which means you will "lose" time by adding 1s on this timer tick - This will accumulate error with every tick. A better approach is to record a start time and use the current time to determine the current offset from that known start time, so that you don't accumulate any error in your time keeping. (There will still be some error, but you will not drift out of touch with real-time over time)
Different timers (Forms.Timer, Thread.Timer etc) will give different accuracies as well - Forms.Timer is particularly poor for accuracy.
You could also use a high performance time to keep track of the time better - see here, for example.

Here is a reliable 1 μs Timer
See https://stackoverflow.com/questions/15725711/obtaining-microsecond-precision-using-net-without-platform-invoke?noredirect=1#comment22341931_15725711
I guarantee its faster and more accurate then StopWatch and PerformanceCounters and uses the fractions of a second you have in the time slice wisely!