In the present tense, worst is used to describe something that is the most bad or negative out of a group or set of things. For example:
* This is the worst movie I've ever seen.
* She's the worst actress in Hollywood.
* The weather today is the worst it's been all week.
In the past tense, worst is used to describe something that was the most bad or negative out of a group or set of things in the past. For example:
* That was the worst day of my life.
* They had the worst luck on their trip.
* The economy was at its worst in the 1930s.
Therefore, worst can be used in both the past and present tense, depending on the context of the sentence.