time of day vs uncertain period

time of day

noun
  • A loosely specified period of time, minutes or hours in duration, especially daytime, or point in time. 

  • The time according to the clock. 

  • The greeting appropriate to the time of day. 

uncertain period

noun
  • An unspecified amount of time 

How often have the words time of day and uncertain period occurred in a corpus of books? (source: Google Ngram Viewer )