I would probably make this really explicit:
// Or private, or maybe even public internal static readonly DateTime MagicValueForNullDateTimeInNonNullableColumns = DateTime.MinValue;
Okay, so I’d make the name slightly less wordy, but you get what I mean. It doesn’t really matter much how you initialize that value – it’s clear what you’re trying to do.
(Of course, use a nullable type any time you can for this. I’m assuming the question was asked because you couldn’t do so in certain situations.)