How I manage system date formate dd/MM/yyyy and MM/dd/yyyy?

The only solution is to use the users Culture settings: any system that doesn't is just guessing what the user might have meant. FOr example, there is no system at all which can tell you is the string "10-11-12" means 10th Nov 2012, 11th Oct 2012, or 12th Nov 2012 - all three of these are valid interpretations used somewhere in the world: Europe, USA, and ISO / Japan respectively.

The only way to tell what the user meant is to query his Culture setting, and convert your string to a DateTime object which is always stored as a DateTime so it doesn't need any further reinterpretation.

The really important thing is to ensure that only valid, correctly converted DateTime values reach a database (and preferably in UTC rather than localized): once they get there the original user culture settings are no longer available, and it can be a total nightmare to fix the problems that causes.

Why are you trying to parse the string "testDate"? You're parsing the literal text "testDate", NOT THE CONTENT of the variable testDate!

The code should be the following. Note the quotes have been removed from the call to .Parse().

string testDate="08/24/2023";
DateTime.Parse(testDate)).AddDays(-1).ToShortDateString()

Would something like the following help?

System.Threading.Thread.CurrentThread.CurrentCulture.DateTimeFormat =
  new System.Globalization.DateTimeFormatInfo()
  {
    ShortDatePattern = "yyyyMMdd"
  ,
    LongDatePattern  = "yyyyMMdd HH:mm:ss"
  ,
    ShortTimePattern = "HH:mm"
  ,
    LongTimePattern  = "HH:mm:ss"
  } ;

Quote:

How I manage system date formate dd/MM/yyyy and MM/dd/yyyy?

I would say 'don't', just use DateTime type variables and let the system present the date to user in user's natural format.
Why ? Because sooner or later, you will have to deal with a date like 12/01/2023. Is it January or December ?

To handle date formats like "dd/MM/yyyy" and "MM/dd/yyyy" in C# ASP.NET MVC, you can try to detect the user's preferred Date Format. For instance, you can try to detect the user's preferred date format by examining the CultureInfo of their system or from any user preferences/settings if available.

for example,

CultureInfo userCulture = System.Globalization.CultureInfo.CurrentCulture;
bool isDateFormatDDMM = userCulture.DateTimeFormat.ShortDatePattern.IndexOf("dd") < userCulture.DateTimeFormat.ShortDatePattern.IndexOf("MM");

The isDateFormatDDMM variable will be true if the user prefers "dd/MM/yyyy" and false if they prefer "MM/dd/yyyy".

Parse Dates Based on User's Preferred Format:
After you've detected the user's preferred format, you can parse dates accordingly. Here's an example of how you can parse a date based on the detected format:

string userInput = "09/19/2023"; // This is an example; you can replace it with actual user input.

DateTime parsedDate;
if (isDateFormatDDMM)
{
    if (DateTime.TryParseExact(userInput, "dd/MM/yyyy", CultureInfo.InvariantCulture, DateTimeStyles.None, out parsedDate))
    {
        // Date was successfully parsed in "dd/MM/yyyy" format.
        // You can use parsedDate for further processing.
    }
    else
    {
        // Handle parsing error for "dd/MM/yyyy" format.
    }
}
else
{
    if (DateTime.TryParseExact(userInput, "MM/dd/yyyy", CultureInfo.InvariantCulture, DateTimeStyles.None, out parsedDate))
    {
        // Date was successfully parsed in "MM/dd/yyyy" format.
        // You can use parsedDate for further processing.
    }
    else
    {
        // Handle parsing error for "MM/dd/yyyy" format.
    }
}

So in the code above, initially check the user's preferred format and then use DateTime.TryParseExact to parse the date according to that format. Doing it this way should allow you to handle date formats "dd/MM/yyyy" and "MM/dd/yyyy" based on the user's system or preferences. Hope this helps!