Please help me understand why the indefinite article is used in this case:
"I had a marvellous time!"
The word "time" can be a countable noun referring to a specific event. "In the time of Julius Caesar ..." means the time period when Julius Caesar was a live. "We had a marvelous time" means the event we were at was marvelous, like we went on a fun vacation or had a pleasant date or some such. "It was the best of times; it was the worst of times." There have been many times, i.e. many time periods, and this one was the best in some ways and the worst in others. Etc.