Now is the time for all good men to come to the aid of their country. This sentence is in the present tense. The word "now" is always in the present tense, and the other verbs in this sentence ("is," "time," "come," "aid") are all present tense as well.
Other examples of present tense verbs include:
"I walk to school every day."
"She sits at her desk and studies."
"He likes to play tennis."
"They are going to the movies tonight."
"We were happy to see them."
"I am writing a paper for school."
"You are the best basketball player on the team."
"He is reading a book."
"She is eating an apple."
"They are drinking milk."