Just a comment. With data, its better if there is just a single source of truth and not need to maintain two sets of data manually.
Have you considered using a sync table to bring in the data from the clients table into a table in your app, and simply use lookup fields so that your data is always equal to their data?
Similarly, if there is data where you are the source of truth, enter it in your app and sync across to a table in their system with the single source of truth for the dates.
We may want to have a discussion outside this forum for a different way to look at this problem which will be automatic.