"Arigato-gozaimasu" or "Arigato-gozaimashita" ?

Which is the right phrase to say "thank you" ,
"Arigato gozai-masu" or "Arigato gozai-mashita" ?

There is a difference in tense.
-masu not-past
-mashita past

If it happened yesterday, and there'd been no chance to say "Thank you." ,
you had better to say "Arigato-gozai-mashita".