title: How do I specify invalid time in a time dimension?
body: <p>I am building a time dimension for only time in my data warehouse. I already have a date dimension.</p>
  
  <p>How do I denote an unknown time? In my DimDate dimension, I marked 01/01/1753 as being reserved for unknown dates, but I think a time will be a bit harder. We don't allow NULLs in our fact tables. How do I do this, and what might that row look like?</p>