Final answer:
When measuring time, a specific date like January 1st, 1999 is used to measure a point in time, while a certain period like the year 1999 is used to measure a duration of time.
Step-by-step explanation:
In the context of measuring time, if you want to measure a specific point in time, such as January 1st, 1999, you would use a specific date. This is typically done using a calendar, which helps track the passage of time and allows people to anticipate seasonal changes or commemorate events. On the other hand, if you want to measure a certain duration of time, such as the entire year of 1999, you would use a period of time. This can be done using units like days, weeks, months, or years.