You cannot change the format attached to a variable from observation to obsevation any more than you could change the name of the variable from observation to observation.
If you wanted to record the fact that for the second observation the time part is unknown then you need to store that information into another variable. One way you could do that would be by storing the format that you want to use to display it. Then at some point in the future you could use the PUTN() function to generate the strings that you see in column B.
Or why not store the date and the time into date and time variables instead of combining them into a singel datetime variable? You could then very easily make a user defined format for the time that will display zero as blanks.
You might also be able to create a user written format, most likely requiring a user written function, that will display the datetime value differently when the timepart is zero. You could then attach that new format to your numeric variable.
http://support.sas.com/documentation/cdl/en/proc/70377/HTML/default/viewer.htm#n1eyyzaux0ze5ln1k03gl338avbj.htm
... View more