Seafarer Tax Calculator lets you keep track of days you were outside of the UK and confirm whether your trips are eligible for claiming Seafarer Earnings Deduction or not (or if you are on track for qualifying).

The app displays a tick or cross beside each trip depending on if it qualifies and the "Stats" section displays data about your trips in a table that matches the one provided by the HMRC, which is required to claim your tax back.

You data can also be exported as a CSV file for you to save for your records or email to an accountant.

This app is free with no ads and only needs permission to write to external storage for exporting CSV files.

Update 2019: This app is not currently under development, nor has it been for many years, I'm leaving it up as it appears many are finding it useful. That said, no feature requests will be worked on. Thanks guys