Agnihotra is practised at exactly sunrise and sunset every day. There are several definitions of sunrise and sunset given in the ancient Himalayan tradition of Vedic knowledge. However, we are told that for bringing nutrition to plant life and reducing disease in an area one particular definition should be used in the Agnihotra process. This relates to half disc on horizon and not counting refraction or visibility.
Agnihotra is now practiced by a growing number of thousands of people on all continents. The computers based in different regions which supply free printouts of sunrise/sunset timings utilize only this definition for their program.
In ancient tradition sunrise is described as follows:
“At sunrise the many fires, electricities, ethers and more subtle energies emanating from the sun extend all the way to the Earth and produce a flood effect at those coordinates where the sun is said to rise. It is awesome. The flood enlivens and purifies everything in its path, destroying what is impure in its wake. This torrent of life-sustaining energies causes all life to rejoice. At sunrise that music can be heard. The morning Agnihotra Mantra is the essence of that music. It is the quintessential sound of that flood. At sunset the flood recedes.”
It is interesting to note that the word electricity is used in plural.
If you miss the timings it is not Agnihotra and the healing effect of the ash will be missing.