at once vs sometime

at once

adv

  • At the same time; simultaneously. 

  • Immediately; now; right away. 

sometime

adv

  • At an indefinite but stated time in the past or future. 

adj

  • Occasional. 

  • Former, erstwhile; at some previous time. 

How often have the words at once and sometime occurred in a corpus of books? (source: Google Ngram Viewer )