Harder was known for his contributions to Danish classical music, with a focus on orchestral and chamber works.
He was also a prolific composer of choral music, and his works were performed by various choirs and ensembles throughout Denmark.
He worked with several orchestras and choirs across Denmark and was known for his dedication to promoting Danish music.
His influence extended to music education, where he served as a professor at the Royal Danish Academy of Music, mentoring the next generation of Danish composers and musicians.
His legacy continues through his compositions, which are still performed today, and through the impact he had on his students and the Danish music community.