title: pandas: best practice to create multiple distinguishable types of NaN
body: <p>I have numeric timeseries data in a DataFrame, where individual columns are periodically updated from a live data source.  The non-updated columns are assigned NaN for these latest dates.  I want to distinguish between cases where the data is missing (just n/a for that date) and when it simply wasn't updated (but another column was).  Currently both are NaN</p>
  
  <p>Do people generally use something other NaN?  Preferably with NaN-like properties</p>