When you sign up for a Day One account, you are assigned a User ID. This can be helpful when troubleshooting issues with our support team. Here is how to locate the User ID:
Open Day One > Settings > tap your name at the top > User ID
Open Day One > https://help.dayoneapp.com/day-one-for-android/accessing-the-settings-in-day-one-android > Account > Day One User ID
Click Day One > Preferences (in the file menu) > Sync > User ID
Look for Day One ID.