What is the data type for "Date of birth" field?

When sending data to client. we were getting response like below.

The deserializer has no knowledge of any type that maps to this name. Consider changing the implementation of the ResolveName method on your DataContractResolver to return a non-null value for name 'DateOnly' and namespace ''.'.  Please see InnerException for more details.

The input request is

<a:KeyValuePairOfstringanyType>
<b:key>uor_dateofbirth</b:key>
<b:value i:type="c:DateOnly">01-02-21</b:value>
</a:KeyValuePairOfstringanyType>

Can you please suggest on this?