What is the DateTimeStyles operator in .NET?

DateTimeStyles isn't an operator - it's an enum, and all enums have the | operator. All it does is apply a bitwise | for the two values. It should only be used for flag-based enums. For example:

public enum AccessMode
{
    None = 0,
    Read = 1,
    Write = 2,
    Delete = 4
}

If you use:

AccessMode mode = AccessMode.Write | AccessMode.Delete;

then you'll have a value with an underlying integer value of 6.

Basically, it allows you to specify a single value representing multiple flags within the enum - so for you're example, you're saying "I want the result to be adjusted to UTC, and allow inner whitespace when parsing."