#### Answer

The year inscribed on the cornerstone of the very old building was \[1989\] written in Roman Numerals.

#### Work Step by Step

The Roman Numeral written on the cornerstone of Building was MCMLXXXIX
Converting MCMLXXXIX to Hindu Arabic Numeral, we get \[\begin{align}
& 1000+\left( 1000-100 \right)+50+10+10+10+\left( 10-1 \right)=1000+900+80+9 \\
& =1989
\end{align}\]
Since the building was very old, usually old people used to write the year of making the building but Roman year written on cornerstone was not too old. So, this was the reason for their surprise.
After Observing and calculating, we found the year inscribed on the cornerstone of the very old building was \[1989\] written in Roman Numerals.