Many date format conversion

I need function to standardize date format. I have string with date: 2015-12-01T00:00:00+00:00, 12/31/2018 01:00:00 etc.

I try this code:

public static DateTime ToDateTime(this string date)
        {
            return DateTime.ParseExact(date, "yyyy-MM-dd", System.Globalization.CultureInfo.InvariantCulture);
        }

but I have error:

System.FormatException
  HResult=0x80131537
  Message=The string was not recognized as a valid DateTime element.
  Source=mscorlib
  Ślad stosu:
   w System.DateTimeParse.ParseExact(String s, String format, DateTimeFormatInfo dtfi, DateTimeStyles style)
   w System.DateTime.ParseExact(String s, String format, IFormatProvider provider)
   w InsuranceService.Common.ExtensionMethods.StringExtensionMethods.ToDateTime(String date) w \\Mac\Home\Desktop\StringExtensionMethods.cs:wiersz 10
   w AutoMapper.Internal.DelegateBasedResolver`2.Resolve(ResolutionResult source)
   w AutoMapper.NullReferenceExceptionSwallowingResolver.Resolve(ResolutionResult source)
   w AutoMapper.PropertyMap.<>c.<ResolveValue>b__44_0(ResolutionResult current, IValueResolver resolver)
   w System.Linq.Enumerable.Aggregate[TSource,TAccumulate](IEnumerable`1 source, TAccumulate seed, Func`3 func)
   w AutoMapper.PropertyMap.ResolveValue(ResolutionContext context)
   w AutoMapper.Mappers.TypeMapObjectMapperRegistry.PropertyMapMappingStrategy.MapPropertyValue(ResolutionContext context, Object mappedObject, PropertyMap propertyMap)

How to fix it?

If you have multiple formats and you know the format of all possible formats, you can ParseExact with all of them.

From your example possible formats

string format1 = "yyyy-MM-ddTHH:mm:sszzzz"; //2015-12-01T00:00:00+00:00"
string format2 = "MM/dd/yyyy HH:mm:ss"; //12/31/2018 01:00:00
string format3 = "dd.MM.yyyy HH:mm:ss"; //01.12.2015 00:00:00

possible inputs

string input1 = "2015-12-01T00:00:00+00:00";
string input2 = "12/31/2018 01:00:00";
string input3 = "01.12.2015 00:00:00";

parsing

DateTime result = DateTime.ParseExact(input1, new[] { format1, format2, format3 }, 
                                      System.Globalization.CultureInfo.InvariantCulture, 
                                      System.Globalization.DateTimeStyles.None);

so ParseExact will pick the first valid format and give you a result.