How does the C# DateTime type distinguish between dates and a point in time whose time just happens to be midnight?

Much of the C# code I've seen uses names like start_date with no indication of whether it really is a date (with no timezone), a date (in one particular timezone), or a datetime where the time is significant.

I'm certainly not a C# developer, though my quick reading of the docs suggests that the DateOnly type was only introduced recently in .NET6.