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How to convert a string formatted like 2018-12-27T02:23:29 to Unix Timestamp in C#

I'm assuming I should just parse the string into a DateTime and go from there... But is there a better way of doing this?

You can use the DateTimeOffset struct, which has a ToUnixTimeSeconds (or ToUnixTimeMilliseconds) method you can use:
long unixTimestamp = DateTimeOffset.Parse("2018-12-27T02:23:29").ToUnixTimeSeconds();
If you're curious how it's done, the source is here: https://referencesource.microsoft.com/#mscorlib/system/datetimeoffset.cs,8e1e87bf153c720e

You should parse it to a normal DateTime object using something from the DateTime.Parse/ParseExact family of functions, and then call a method like this:
public int ToUnixTime(DateTime d)
{
var epoch = new DateTime(1970,1,1);
return (int)(d - epoch).TotalSeconds;
}

DateTime interally stores the "Ticks" (a invented Time unit) since "12:00:00 midnight, January 1, 0001 (0:00:00 UTC on January 1, 0001), in the Gregorian calendar". As a Int64/Long Number. DateTime thus beats UnixTime easily in possible values. Converting to DateTime should be lossless.
Any ToString() call, any other Property call will simply calculate the values based on those Ticks (and culture/Timezone settings for ToString()). Everything else is just a interpretation of the Tick value.
You should parse to DateTime. And getting from Ticks to something as inprecise as the UnixTime is easy math. See Joels Answer for that.
Do note however the DateTimes preccision and accuaracy do not match fully: https://blogs.msdn.microsoft.com/ericlippert/2010/04/08/precision-and-accuracy-of-datetime/ DateTime.Now will usually give you only return values in 18 ms steps. And even the Stopwatch has issues with values < 1 ms.

How to remove time in date time?

How to remove a time in date time ? on column date its only display format
I store the value on repository combobox dropdown, and it store the value including the time. How do I remove the time?
I know there's so many question about this. But the solution was by converting it into a date.tostring("dd MMM yyyy"). Is there a solution beside convert it into string? I want the value was date time not a conversion of string.
The code I am using still giving me a time.
DateTime date = Convert.ToDateTime(gridView1.GetDataRow(i)["date"]);

You just forgot to specify the date at the end of the conversion
DateTime date = Convert.ToDateTime(gridView1.GetDataRow(i)["date"]).Date;

DateTime as the name implify, stores date and time.
You cannot remove time part from date because time is an integral part of date.
To understand this you will have to understand how the date and time are stored. Internally, the date and time is stored as a rational number (in fractions). In computer system 24 hours are considered as numeric 1, so when your value is increased by 1 that means your date is increased by 1 day. If the value is increased by 0.5 that means your date is increased by 12 hours (half day).
So, when you have value 42613.00 that means 31st August at midnight (just when the day started) and if you have value 42613.25 that means 6 AM of 31 Aug 2016 and 42613.50 means 12 noon of 31 Aug 2016 (and 42613.39236 means 9:25:00 AM of 31 Aug 2016)
The smallest fraction of time that need to be stored is 1 millisecond. That means the values of DateTime field should have a precision of more than 0.0000000115740740740741. But this is an irrational value (in binary) and hence cannot be stored as such (the nearest match is 1.00000000000000000000000000110001101101011101010000111010111111..., ... means there are more), so I can say that milliseconds are to their nearest approximation values.
.
That said,
if you wish to take only Date part, you can create your own class or struct to store date part of the DateTime and then override operators for date arithematic and provide implicit conversions to convert them to DateTime if any code that expect DateTime field.

How to convert a double value to a DateTime in c#?

I have the value 40880.051388 and am storing it as a double, if I open Excel and paste in a cell and apply the following custom format "m/d/yyyy h:mm" to that cell, I get "12/3/2011 1:14"
How can I do this parsing/Conversion in C#? I don't know if the value is milliseconds from a certain checkpoint, like epoch time, or if the value is in some specific prepared format, but how does excel come up with this particular value? Can it be done in C#?
I've tried working with TimeSpan, DateTime, and other like things in Visual Studio but am not getting anywhere.

Looks like you're using the old OLE Automation date. Use
DateTime.FromOADate(myDouble)

Try something like this:-
double d = 40880.051388 ;
DateTime dt = DateTime.FromOADate(d);

Try using var dateTime = DateTime.FromOADate(40880.051388);.
If you need to format it to a string, use dateTime.ToString("M/d/yyyy H:mm", CultureInfo.InvariantCulture) for that. That will give you 24-hour string (change H to h for a 12-hour system).
If you need greater precision (by a factor 1000 or more) than offered by FromOADate, see my answer in another thread.

The value is an offset in days from December 30th, 1899. So you want:
new DateTime(1899, 12, 30).AddDays(40880.051388)

The following simple code will work
DateTime.FromOADate(myDouble)
However if performance is critical, it may not run fast enough. This operation is very processor intensive because the range of dates for the OLE Automation Date format begins on 30 December 1899 whereas DateTime begins on January 1, 0001, in the Gregorian calendar.
FromOADate calls a DoubleDateToTicks function using myDouble as the only argument. This returns the number of ticks, and this value is used to create a new DateTime with unspecified DateTimeKind.
The vast bulk of this work is done by the DoubleDateToTicks function in mscorlib. This includes code to throw an ArgumentException when the value of the double is NaN, and there are numerous ways in which it can be performance optimized depending on your exact needs.

SQLite Date and Time Datatype

I am trying to build a nice, small database to run on a mobile application (Windows Mobile 5, if you are curious).
In the SQLite Documentation, the Date and Time Datatype is defined as follows:
1.2 Date and Time Datatype
SQLite does not have a storage class set aside for storing dates
and/or times. Instead, the built-in Date And Time Functions of SQLite
are capable of storing dates and times as TEXT, REAL, or INTEGER
values:
TEXT as ISO8601 strings ("YYYY-MM-DD HH:MM:SS.SSS").
REAL as Julian day numbers, the number of days since noon in Greenwich on November 24, 4714 B.C. according to the proleptic
Gregorian calendar.
INTEGER as Unix Time, the number of seconds since 1970-01-01 00:00:00 UTC.
Applications can chose to store dates and times in any of these
formats and freely convert between formats using the built-in date and
time functions.
So, saving my DateTime value as either a REAL (float) or INTEGER is the same size.
What about the TEXT format? There are 23 characters above in the text YYYY-MM-DD HH:MM:SS.SSS. Is that 8-bytes per character? If so, that is a HUGE waste of space to store in Text format (which is what I am currently doing).
What about the REAL format? Would I define a base date of November 24, 4714 B.C.? (I am not even sure if Visual Studio 2008 will let me do that. I've never tried.) Then get the TimeSpan between base date and date I want, extract the number of days, and store that?
// is this how to declare this date?
private static readonly DateTime nov24_4714bc = new DateTime(-4714, 11, 24);
public static double GetRealDate(DateTime dateTime) {
// FYI: subtracting dates in .NET returns a time span object
return (dateTime - nov24_4714bc).TotalDays;
}
What about the INTEGER format? Would I define a base date of 1970-01-01 00:00:00 UTC (please tell me how to do that!), then get the TimeSpan between base date and my input date, extract the number of seconds, and store that?
// is this a UTC date?
private static readonly DateTime utc1970_01_01 = new DateTime(1970, 1, 1);
public static double GetIntDate(DateTime dateTime) {
// FYI: subtracting dates in .NET returns a time span object
return (dateTime - nov24_4714bc).TotalSeconds;
}
Any help with this? I am a little confused on a few points.

Use the TEXT format if "human-readability" is important.
Use one of the numeric formats if saving space is important.
If you don't need millisecond precision, you can save space in the TEXT format by only including the part you do need. There are 3 shorter formats accepted by SQLite date/time functions:
YYYY-MM-DD HH:MM:SS (19 characters)
YYYY-MM-DD HH:MM (16 characters)
YYYY-MM-DD (10 characters)
(NEVER use MM/DD/YYYY; it's not supported, and it doesn't sort correctly.)
Would I define a base date of November 24, 4714 B.C.? (I am not even
sure if Visual Studio 2008 will let me do that. I've never tried.)
You can't: System.DateTime only supports the years 1 to 9999. You need to pick a different base date, and then do (dateTime - baseDate).TotalDays + baseDateJD, where baseDateJD is the Julian date of the base date. Some reasonable choices are:
0001-01-01 = JD 1721425.5
1970-01-01 = JD 2440587.5
2000-01-01 = JD 2451544.5

DateTime Format like HH:mm 24 Hours without AM/PM

I was searching here about converting a string like "16:20" to a DateTime type without losing the format, I said I dont want to add dd/MM/yyy or seconds or AM/PM, because db just accept this format.
I tried with Cultures yet
Thanks in Advance

Just give a date format to your dateTime.
string DateFormat = "yyyy MM d " this willl give you the year month and day. after continuing;
string DateFormat = "yyyy MM d HH:mm:ss " in here the Capital H will give you the 24 hours time format and lowerCase "h" will give you the 12 hours time format...
when you give the Dateformat as a string you can do whatever you want with date and time.
string DateFormat = "yyyyMMdHHmmss";
string date = DateTime.Now.ToStrign(DateFormat);
OR
Console.writeline(DateTime.Now.ToStrign(DateFormat));
OUTPUT:
20120823132544

All DateTime objects must have a date and a time.
If you want just the time, use TimeSpan:
TimeSpan span = TimeSpan.Parse("16:20");
If you want a DateTime, add that time to the min value:
TimeSpan span = TimeSpan.Parse("16.20");
DateTime dt = DateTime.MinValue.Add(span);
// will get you 1/1/1900 4:20 PM which can be formatted with .ToString("HH:mm") for 24 hour formatting

DateTime.Now.ToString("hh:mm") - If it's C#.
Oh. Only read the header.
DateTime dt = new DateTime(2008, 12, 11, Convert.ToInt32("16"), Convert.ToInt32("32"), 0);

what do you mean by "losing the format".
if you convert it to a DateTime type, then the DateTime object will have dd/mm/yy and other properties. depending on how you plan to use the object, you can "recover" your original settings, by formatting the string output like this: DT.ToString("HH:mm");

Since you don't stipulate which DBMS you are using, it is hard to know which answer will help you. If you use IBM Informix Dynamic Server, you would simply use the data type 'DATETIME HOUR TO MINUTE', which will record values in the 24 hour clock.

DateTime.Parse("16:20")

I want to address this part of your question:
without losing the format
A database will generally store all datetime values in a standard common format that's not even human readable. If you use a datetime column the original format is destroyed.
However, when you retrieve the value you cast it back to any format you want. If you want HH:mm you can get it.