The start time of a century depends on the chosen reference point or calendar system. Traditionally, centuries are defined based on the Gregorian calendar, which is the most widely used calendar system today. According to the Gregorian calendar, a new century begins at the start of a year whose last two digits are "01."
For example, the 21st century began on January 1, 2001, because the year 2001 is the first year of the 21st century. Therefore, the start time of the 21st century in terms of hours, minutes, seconds, and milliseconds would be 00:00:00 (midnight) on January 1, 2001.
To specify the start time down to milliseconds, we can consider it as 00:00:00.000. However, it's worth noting that milliseconds are not typically part of standard date and time formats in everyday usage, as they represent very small fractions of a second and are often unnecessary for most practical purposes.
Please keep in mind that the above explanation is based on the Gregorian calendar, which is widely used but not the only calendar system. Other calendar systems, such as the Julian calendar or different cultural calendars, may have different conventions for determining the start of a century.