System.FormatException: String '638143033743100000' was not recognized as a valid DateTime

Error:

System.FormatException: String '638143033743100000' was not recognized as a valid DateTime

Current code already has provision to allow two different date format:

items = (await itemService.GetItems())
       .GroupBy(x => x.ItemKey)
       .Select(x => new DisplayItem
       {
           item = x.item,            
           country = x.FirstOrDefault(y => y.item == "Country")?.Value,             
           date = x.FirstOrDefault(y => y.item == "Date")?.Value != null
           ? DateTime.ParseExact(x.FirstOrDefault(y => y.item == "Date")?.Value, 
           new string[] {
            "yyyy-MM-dd HH:mm:ss.FFFFFFF",
            "dd.MM.yyyy HH:mm:ss"}, null, DateTimeStyles.AssumeLocal)
           : (DateTime?)null
       }).Where(s => s.country!= null).OrderByDescending(s => s.date).ToList();       

We have added a new date format. Using time in ticks now. May I know the format for ticks. Thank you.

EDIT: Didn't see it was in ticks and not in UnixTime

To convert from string ticks to DateTime, parse it as a long and use the following

long ticks = 638143033743100000L;
DateTime dt = new(ticks);

Here is a working fiddle

And here is how you can use it in your code

items = (await itemService.GetItems())
   .GroupBy(x => x.ItemKey)
   .Select(x => new DisplayItem
   {
       item = x.item,            
       country = x.FirstOrDefault(y => y.item == "Country")?.Value,             
       date = (x.FirstOrDefault(y => y.item == "Date") != null) && long.TryParse(x.FirstOrDefault(y => y.item == "Date").Value, out long l) && l > 0
       ? new DateTime(l) : (DateTime?)null
   }).Where(s => s.country!= null).OrderByDescending(s => s.date).ToList();  

You can then set the formatting on your property date later down the line.

Original answer:

Here is how to convert a Timestamp to a DateTime

First, convert your string to a long

If it is in seconds

DateTimeOffset.FromUnixTimeSeconds(long).UtcDateTime

If it is in milliseconds

DateTimeOffset.FromUnixTimeMilliseconds(l).UtcDateTime;

It if better to make an extension method

public static class NumericExtensions
{
    public static DateTime ConvertFromTimestampSecondsToDateTime(this long l) => DateTimeOffset.FromUnixTimeSeconds(l).UtcDateTime;
    public static DateTime ConvertFromTimestampMillisecondsToDateTime(this long l) => DateTimeOffset.FromUnixTimeMilliseconds(l).UtcDateTime;
}

Then use them like so

long timestamp = 638143033743100000l;
DateTime dt = timestamp.ConvertFromTimestampMillisecondsToDateTime();

Create a new method and use that instead of ParseExact directly and call that instead. Should make your code even more readable. Your handling of null cases in the midth of the linq statement is a maintenance mess.

.Select(x => new DisplayItem
   {
       item = x.item,            
       country = x.FirstOrDefault(y => y.item == "Country")?.Value,             
       date = MyLovelyDateTimeParse(x.FirstOrDefault(y => y.item == 
       "Date")?.Value),
       ...... etc.
   })



public DateTime? MyLovelyDateTimeParse(string dateString)
{
	if (String.IsNullOrEmpty(dateString))
		return null;
	else if (long.TryParse(dateString, out long value))
		return new DateTime(value);
	else
		return DateTime.ParseExact(dateString,
			new string[] {
			"yyyy-MM-dd HH:mm:ss.FFFFFFF",
			"dd.MM.yyyy HH:mm:ss"}, null, DateTimeStyles.AssumeLocal);			
}