To find a relative date 100 days in the future from a specific date, such as 30th December, you can manually calculate it by adding 100 days to the given date. In this case, starting from 30th December, a straightforward way to find the future date is to add 100 days, which would lead to 9th April in the following year.

To calculate this manually, you can first identify the month lengths. Then, starting from 30th December, you would exhaust the days in December (1 day), then the entirety of January (31 days), February (28 or 29 days depending on leap year), and finally part of March (up to 9 days). By adding these, you arrive at the 9th of April, the future date.

This method involves simple addition, where you progressively add up the days from the initial date until you reach the 100-day mark. This manual calculation provides insight into how the days flow through the months, accounting for the varying lengths of different months and leap years. It exemplifies the fundamental principle of adding days to determine future dates accurately.

By following this manual calculation, you can gain a deeper understanding of how time increments operate within the Gregorian calendar. This process showcases the relationship between days, months, and years, allowing you to appreciate the intricacies of time reckoning and how simple arithmetic can be used to determine future dates. This method illuminates the practical application of basic mathematical operations in the context of time management and planning.