how do I get portfolio loss/gain over a time period e.g. calendar or tax year?
1 Set up a Google spreadsheet with the contents of every trading account in it.
2 Set a Google calendar reminder to download at periodic intervals (e.g monthly). Download as csv file to get the data rather than any formulae.
3 Put the summary data in an Excel spreadsheet with (for example), one column per month.
Before you know it, you will have several years of data.
