The 21st century: naval encounters, airline reports, radar/IR videos, government disclosures, and mass sightings captured by millions.
Before 3300 BCE
3300 BCE - 500 CE
500 BCE - 500 CE
500 CE - 1500 CE
1500 CE - 1800 CE
1800 CE - 1900 CE
1900 CE - 2000 CE
2000 CE - Present