I posted here a brief summary of how I export from Day One into DEVONThink and, on the Hazel forums, a more detailed description of the role of Hazel in that process.
The only disadvantage of that process is that Day One tags are not exported but you can overcome that by using hastags in Day One and then use the DT option to convert hashtags to tags on import. I did at one stage use Bear but find my current process much speedier and more reliable.
Talking of reliability, I have used Day One for seven years but, on several occasions, had horrendous problems with the Day One sync service. I eventually found that so unreliable (e.g., in not picking up minor corrections to a number of old entries and then stopping working altogether) that I’ve stopped using Day One sync and simply now create all entries on my Mac. If at any time in future I have to use my iPad to create entries I’ll work out at the time how to transfer those few entries to my Mac—even if it involves some rather crude cutting and pasting! I’m not unduly concerned about that as my Mac travels with me 99% of the time.
The beauty of having all diary entries in DT is the wonderful search facility (which I use on my diary entries several times every day), the ability to add photos without slowing down the app and, perhaps above all, the ability to script (fe.g., to replace Day One links—to other entries—with DT links).
Stephen