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Combine date and time from string and convert to datetime

I have 2 strings (date(13/04/2021),time("06:30")) and I want to combine them together to datetime format and I am getting the below error.
System.FormatException: 'String was not recognized as a valid DateTime.'
What I am doing wrong?
if I change the tempdate format to "yyyy-dd-MM" I get the error there
var type = form["GymType"];
var time = form["states_ddl"];
var date = form["date2"];
var username = form["username"];
var numberofpersons = 0;
if (type == "2")
{
//gym
numberofpersons = 9;
}
else
{
//cross
numberofpersons = 5;
}
Booking toadd = new Booking();
var currenduser = User.Identity.GetUserId();
var tempdate = DateTime.ParseExact(date , "dd/MM/yyyy", CultureInfo.InvariantCulture);
DateTime d = DateTime.ParseExact(tempdate + " " + time, "yyyy-dd-MM HH:mm", CultureInfo.InvariantCulture);

What you do is convert de string to a dateTime 'tempdate', and than on the next line you convert it back to a string.
What i would do is convert the date as you do and parse the time separate, than add them together.
var tempdate = DateTime.ParseExact(date , "dd/MM/yyyy", CultureInfo.InvariantCulture);
var timspan = TimeSpan.Parse(time);
DateTime d = tempdate.Add(timspan);

You can use Convert to do this
var date = Convert.ToDateTime("13/04/2021 06:30");

try this
string d = "13/04/2021";
string t = "10:30PM";
string dateAndTime = d.Trim() + ' ' + t.Trim();
DateTime dt = DateTime.ParseExact(dateAndTime, "MM/dd/yyyy hh:mmtt",
CultureInfo.InvariantCulture, DateTimeStyles.None);
dt = DateTime.Parse(dateAndTime, CultureInfo.InvariantCulture,
DateTimeStyles.None);

String which is not in datetime format to DateTime

I have a string which is not in a datetime format eg:20160503. How can i change it to Datetime. I tried using Substring. It is working.Is there any more efficient way? Below is what I have right now.
string a = "20160503";
int b = Convert.ToInt32(a.Substring(0, 4));
int c = Convert.ToInt32(a.Substring(4, 2));
int d = Convert.ToInt32(a.Substring(6, 2));
string date = b + "/" + c + "/" + d;
DateTime result = new DateTime();
DateTime.TryParse(date, out result);

Since you know the exact format of your datetime, you could try to use the ParseExact DateTime's method.
var dt = DateTime.ParseExact(a,"yyyyMMdd",CultureInfo.InvariantCulture);
For further info, please have a look here.

Try somthing like this:
Define your own parse format string to use.
string formatString = "yyyyMMdd";
string sample = "20160503";
DateTime dt = DateTime.ParseExact(sample,formatString,null);

Thanks for your replies. Finaly I ended up using DateTime.TryParseExact
string dateString = "20150503";
DateTime dateValue = new DateTime();
DateTime.TryParseExact(dateString, "yyyyMMdd", new CultureInfo("en-US"), DateTimeStyles.None, out dateValue);

convert C# date time to string and back

I'm converting C# date time to string. Later when I convert it back to DateTime object it appears that they are not equal.
const string FMT = "yyyy-MM-dd HH:mm:ss.fff";
DateTime now1 = DateTime.Now;
string strDate = now1.ToString(FMT);
DateTime now2 = DateTime.ParseExact(strDate, FMT, CultureInfo.InvariantCulture);
Console.WriteLine(now1.ToBinary());
Console.WriteLine(now2.ToBinary());
Here is the example. Looks like everything is included in string format, when I print date both displays the same, but when I compare objects or print date in binary format I see the difference. It looks strange to me, could you please explain what is going on here?
Here is the output for the code above.
-8588633131198276118
634739049656490000

You should use the roundtrip format specifier "O" or "o" if you want to preserve the value of the DateTime.
The "O" or "o" standard format specifier represents a custom date and time format string using a pattern that preserves time zone information. For DateTime values, this format specifier is designed to preserve date and time values along with the DateTime.Kind property in text. The formatted string can be parsed back by using the DateTime.Parse(String, IFormatProvider, DateTimeStyles) or DateTime.ParseExact method if the styles parameter is set to DateTimeStyles.RoundtripKind.
Using your code (apart from changing the format string):
const string FMT = "O";
DateTime now1 = DateTime.Now;
string strDate = now1.ToString(FMT);
DateTime now2 = DateTime.ParseExact(strDate, FMT, CultureInfo.InvariantCulture);
Console.WriteLine(now1.ToBinary());
Console.WriteLine(now2.ToBinary());
I get:
-8588633127598789320
-8588633127598789320

Oded's answer is good but it didn't work for me for UTC dates. In order to get it to work for UTC dates, I needed to specify a DateTimeStyles value of "RoundtripKind" so that it didn't try to assume it was a local time. Here is the updated code from above:
const string FMT = "O";
DateTime now1 = DateTime.Now;
string strDate = now1.ToString(FMT);
DateTime now2 = DateTime.ParseExact(strDate, FMT, CultureInfo.InvariantCulture, DateTimeStyles.RoundtripKind);
Console.WriteLine(now1.ToBinary());
Console.WriteLine(now2.ToBinary());
Note, this works for both UTC and local dates.

2 things:
You can use the ParseExact overload that takes a DateTimeStyle parameter in order to specify AssumeLocal.
There will still be a small difference between now1 and now2 unless you increase the precision to 7 digits instead of 3: "yyyy-MM-dd HH:mm:ss.fffffff"
const string FMT = "yyyy-MM-dd HH:mm:ss.fffffff";
DateTime now1 = DateTime.Now;
string strDate = now1.ToString(FMT);
DateTime now2 = DateTime.ParseExact(strDate, FMT, CultureInfo.InvariantCulture, DateTimeStyles.AssumeLocal);
Console.WriteLine(now1.ToBinary());
Console.WriteLine(now2.ToBinary());
Even without the above changes, the calculated difference between now1 and now2 appears small, even though the binary values do not appear similar.
TimeSpan difference = now2.Subtract(now1);
Console.WriteLine(difference.ToString());

If you dont need the string to be human-readable (like, you want to cipher it before storage), you can just call string str = dt1.ToBinary().ToString(); and DateTime dt2 = DateTime.FromBinary(long.Parse(str));
DateTime now1 = DateTime.Now;
string strDate = now1.ToBinary().ToString();
DateTime now2 = DateTime.FromBinary(long.Parse(strDate));
Console.WriteLine(now1.ToBinary());
Console.WriteLine(now2.ToBinary());

Just use this code it convert date time to string and string to date time
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
namespace DateTimeConvert
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
private void button1_Click(object sender, EventArgs e)
{
label1.Text = ConvDate_as_str(textBox1.Text);
}
public string ConvDate_as_str(string dateFormat)
{
try
{
char[] ch = dateFormat.ToCharArray();
string[] sps = dateFormat.Split(' ');
string[] spd = sps[0].Split('.');
dateFormat = spd[0] + ":" + spd[1] + " " + sps[1];
DateTime dt = new DateTime();
dt = Convert.ToDateTime(dateFormat);
return dt.Hour.ToString("00") + dt.Minute.ToString("00");
}
catch (Exception ex)
{
return "Enter Correct Format like <5.12 pm>";
}
}
private void button2_Click(object sender, EventArgs e)
{
label2.Text = ConvDate_as_date(textBox2.Text);
}
public string ConvDate_as_date(string stringFormat)
{
try
{
string hour = stringFormat.Substring(0, 2);
string min = stringFormat.Substring(2, 2);
DateTime dt = new DateTime();
dt = Convert.ToDateTime(hour+":"+min);
return String.Format("{0:t}", dt); ;
}
catch (Exception ex)
{
return "Please Enter Correct format like <0559>";
}
}
}
}

take time and append to date to make datetime?

Say I have the following:
string fromTime = "8:00am";
string someDate = "06/01/2012";
I want to end up doing this:
DateTime dt = Convert.ToDateTime(someDate + someTime);
Basically I want to take a date and add the time to it with the result for the above to be
06/01/2012 8:00am
But have it stored in a datetime variable. Is this possible?

You could use the DateTime.TryParseExact method which allows you to specify a format when parsing:
class Program
{
static void Main()
{
string s = "06/01/2012 8:00am";
string format = "dd/MM/yyyy h:mmtt";
DateTime date;
if (DateTime.TryParseExact(s, format, CultureInfo.InvariantCulture, DateTimeStyles.None, out date))
{
Console.WriteLine(date);
}
}
}

Try this:
DateTime dt = DateTime.ParseExact(someDate + " " + someTime, "MM/dd/yyyy h:mmtt", CultureInfo.InvariantCulture);

how to change time format to 24hrs? in c#

Literal four = new Literal();
string timeanddate;
timeanddate = DateTime.UtcNow.ToString();
DateTime dt = new DateTime();
DateTime dt_calc = new DateTime();
dt = Convert.ToDateTime(timeanddate);
dt_calc = dt.AddHours(3);
four.Text = "3hr added and this gives>> " + dt_calc.ToString();
form1.Controls.Add(four);
its all in AM PM i want to work with 24hrs

See this page for every way you could possibly want to format a DateTime.
Note that you use "HH" for 24-hour time.
For example, if you wanted the format "23:00:00" instead of "11:00:00 PM" you would use:
string formatted = dt_calc.ToString("HH:mm:ss");
By the way, your initialization of your DateTime values with new DateTime() is unnecessary.

You have to change the current culture.
System.Threading.Thread.CurrentThread.CurrentCulture = new CultureInfo(1053);
string swedishTime = DateTime.Now.ToShortTimeString(); //24h format
System.Threading.Thread.CurrentThread.CurrentCulture = new CultureInfo(1033);
string englishTime = DateTime.Now.ToShortTimeString(); //am/pm format

System.Threading.Thread.CurrentThread.CurrentCulture.DateTimeFormat.ShortTimePattern = "HH:mm:ss"

Old post, but here is the syntax from above with string interpolation:
string formatted = $"{dt_calc:HH:mm:ss}";