Simplest alternative to CSV for a developer to produce?

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If you have programming skills and intend to import data from another program to Manager, use batch update (batch create to determine the format) as described here

How to batch import in manager

Some hints for batch creating. Most tab in Manager correspond to a different relational database tables The required data format for each tabs batch create is most easily determined by manually entering some data in that tab then doing a “Batch update”. Record links to other tables is done via a UUID. The UUID for most of theses links can be easily found by doing a batch update of the target database table / tab, the final field is the Key / UUID for that item. Exceptions include Tax…

The official position is, don’t do it, retain your old system for old data, for new data set a start date and starting balances.

Why you may want to do it anyway is discussed here

Can I import data from Quicken?

The advantage is it enables you to: Correct the deficiencies in your old accounting package (their must be a reason you are moving from the old accounting package). Add the new functionality available in Manager to your old records Test Manager with your real live data prior to training your staff or using it for several years to accumulate a significant amount of new data. Show historical comparison data in all your reports. Many users (but not all) find this capability useful …