Now my problem is that I need to keep a separate file for each period
Why split this data into separate files? There are many ways to manage your data by time period without needing to split the data up like this and doing so will greatly complicate managing your data--this post being just the first example of that.
Thank you for your quick comment. Sorry it took so long to reply to your suggestion.
The reason why data is split into separate files is that I inherited this job from a former colleague who kept each time period in a separate file.
I would really be grateful if you could tell me how to better organise the data so I don't hit snags as the one described above.As always, thanks in advance for your assistance,Otto
You'll need to analyze your current design to figure out why it was split this way in the first place. That's not something I can do from here.
You might take a copy of your system and see what happens if you import data to combine data from two or more years in the same file and then experiment with this copy to see what issues develop. Generally speaking, you can set up scripts, relationships, portal filters, and so forth that can limit the records shown by the year so that you can see only the current year's data or only the data from a year specified by you.