I am very confused about the timeline between Valmiki and the creation of Ramayana. I have read somewhere that Valmiki existed during the time period of 100 BCE, and actually met Lord Ram in person. This is impossible, however, because Ramayana took place thousands of years before this. Is there an error with what I read? Can anybody provide an accurate timeline and the relation between Valmiki and Ramayana.
Valmiki was contemporary to Sri Rama. As per Matsya Purana, Ramayana happenned during 24th Treta Yuga of Vaivasvara (current) Manvantara. Valmiki composed the story of Rama through meditation as described in Chapter 2 and 3 of Balakanda. Valmiki composed not only past and present of Rama's life but also future work of Rama in advance. This clearly shows Valmiki was contemporary to Sri Rama.
As per puranas, chaturyuga makes a period of 12,000 divine years. One must multiply divine years with 360 to convert to Human years. Different people differ on multiplication with factor 360. But traditional calculation puts date of Ramayana some lakhs of years ago.
Coming to 100BCE, this date might be given by some western Indologist. They date the available manuscripts and infer some date through their dating techniques. They even dated Rigveda manuscripts and fixed a date for its composition, completely neglecting oral tradition of transmission of Shruti or Vedas. So, you can consider their dates with a grain of salt.