C#

public static string TimeConverter(string time)
{
    if (!DateTime.TryParseExact(time, "h:mmtt", CultureInfo.InvariantCulture, DateTimeStyles.None, out var value))
    {
        return string.Empty;
    }
    
    return value.ToString("HH:mm:ss", CultureInfo.InvariantCulture);
}

Pete O'Hanlon wrote:

you wouldn't store numbers as strings would you?

System.Globalization.PersianCalendar

DateTime dt = new DateTime(1400, 6, 7);

string fmt = dt.ToString("yyyy/MM/dd");

// => 1400/06/07

Richard Deeming wrote:

The ToString call will allocate a string on the heap. But if you want to display the date to the user, then it has to be converted to a string at some point.

#if NETSTANDARD2_0
namespace System.Buffers
{
    public delegate void SpanAction<T, in TArg>(Span<T> span, TArg arg);
}
#endif

static class StringHelpers
{
    public static string Create<TArg>(int length, TArg state, SpanAction<char, TArg> action)
    {
        if (length < 0) throw new ArgumentOutOfRangeException(nameof(length));
        if (action is null) throw new ArgumentNullException(nameof(action));
        if (length == 0) return string.Empty;

#if NETSTANDARD2_0
        unsafe
        {
            var str = new string('\0', length);
            fixed (char* chars = str)
            {
                var span = new Span<char>(chars, length);
                action(span, state);
            }

            return str;
        }
#else
        return string.Create(length, state, action);
#endif
    }
}

public static string FormatDateTime(DateTime dt) => StringHelpers.Create(21, dt, static (buffer, date) =>
{
    int value = date.Day;
    buffer[0] = (char)('0' + (value / 10));
    buffer[1] = (char)('0' + (value % 10));
    
    buffer[2] = '.';
    
    value = date.Month;
    buffer[3] = (char)('0' + (value / 10));
    buffer[4] = (char)('0' + (value % 10));
    
    buffer[5] = '.';
    
    value = date.Year;
    buffer[6] = (char)('0' + ((value / 10) % 10));
    buffer[7] = (char)('0' + (value % 10));
    
    buffer[8] = ' ';
    
    value = date.Hour;
    buffer[9] = (char)('0' + (value / 10));
    buffer[10] = (char)('0' + (value % 10));
    
    buffer[11] = ':';
    
    value = date.Minute;
    buffer[12] = (char)('0' + (value / 10));
    buffer[13] = (char)('0' + (value % 10));
    
    buffer[14] = ':';
    
    value = date.Second;
    buffer[15] = (char)('0' + (value / 10));
    buffer[16] = (char)('0' + (value % 10));
    
    buffer[17] = ':';
    
    value = date.Millisecond;
    buffer[18] = (char)('0' + ((value / 100) % 10));
    buffer[19] = (char)('0' + ((value / 10) % 10));
    buffer[20] = (char)('0' + (value % 10));
});

public static string Create<TState> (int length, TState state, System.Buffers.SpanAction<char,TState> action);