Conversion from milliseconds to DateTime format

DateTime in .NET is initialized to 0001-01-01 00:00:00 and then you add your TimeSpan, which seems to be 45 Years.

It is common for such (milli)-second time definitions to start at 1970-01-01 00:00:00, so maybe the following gives you the expected result:

double ticks = double.Parse(startdatetime);
TimeSpan time = TimeSpan.FromMilliseconds(ticks);
DateTime startdate = new DateTime(1970, 1, 1) + time;

or simply

var date = (new DateTime(1970, 1, 1)).AddMilliseconds(double.Parse(startdatetime));

The reason is that your value is based on milliseconds elapsed since 01/01/1900 or 01/01/1970 and DateTime in C# starts with 01/01/00001.

I think it starts from 01/01/1970 because 1970 + 45 would be 2015 which I think it is the year you search.

Since TimeSpan.Ticks property returns long, your new DateTime(time.Ticks) code call DateTime(long) constructor and from it's documentation;

A date and time expressed in the number of 100-nanosecond intervals that have elapsed since January 1, 0001 at 00:00:00.000 in the Gregorian calendar.

That's why it's wrong to say The result is almost good. The value of result is expected as implemented and documented.