To find a relative date, 18 months in the future from 29th December, we can manually calculate the new date by adding 18 months to the current date. Firstly, convert 18 months into days. Since a month can have varying days, we need to consider this variability in our calculation. On average, a month is 30.44 days. So, 18 months will be approximately 548 days.

Next, we add these 548 days to the 29th of December. Starting with December, we have 2 days left in the month, which takes us to 31st December. Moving into the next year, we will have 365 days.

Therefore, by adding the remaining days of December, the full 365 days of the following year, and then the remaining 181 days after that, we arrive at the 29th of June in the subsequent year as the relative date, 18 months in the future from the 29th of December.