At the least you should not use "year" in the way you have; "longest year" is confusing as it suggests that the length of a year changes substantially. A possible alternative would be:
"to the longest period with reliable documentary data"
Some re-structuring of your sentence might make your meaning more explicit, though. For example:
"Previous analyses have been limited to a research period of one year, by a lack of observatory data; this novel method allows the research period to be extended over a much longer timescale (100 years in the present study) determined by the earliest date at which reliable data is available."
I'd comment though, that this wording might be further improved/corrected, but I am not clear from your question of a couple of points of necessary information:
- What is the difference between the "observatory data" which is missing for periods >1 year, and the "reliable data" which is available from 100 years back?
- Does the "reliable data" just have to be available from some time in the past, or does your method need a continuous run of data for the 100 year period?
I am assuming this is a concluding summary in a technical paper - and of course it is important to be specific and accurate in such a situation.