I am using simplewallet. Is there some way to prove how long these funds have been in my account?
Question adapted from here.
By using simplewallet, and running
show_transfers command, you will get a list of all transactions, with their corresponding block height and transaction IDs. Block height is like a timestamp, and by using a block explorer, we can easily find when was that block found (example). You could also fetch the details yourself by running bitmonerod and using the command
print_block <n>, which would give you details of the block with block height n. This way, you don't have to trust a centralized service. The resulting timestamp will be printed as epoch Unix time, which can be converted to human-readable DateTime.
However, the above doesn't really prove it to anyone except yourself because you if you just c&p the simplewallet log to some auditor as "proof", there's nothing preventing you from fabricating the result.
To really prove when you received certain funds, you'd have to give up your view key to the auditor. He could then create a watch-only wallet by using simplewallet and by using the
show_transfers command, see only the incoming transactions. However, this doesn't tell him if you spent those funds in the meantime.
To prove that you still have the funds, or to show when you've spent them, you'd then need to export the key images and hand them over to the auditor who would then import them into his watch-only wallet created with your viewkey. This was described here, but the function to import/export key images has not yet made it to the official release. It could be made available by compiling latest repo from source, though.
show_transfers command in simplewallet will give you all transaction IDs of all incoming and outgoing transfer of a certain wallet. Subsequently, you can look up those transaction IDs on the blockchain to see in which block they were included. Those blocks also have a timestamp and as such you know the exact time the transaction was included into a block and thus had it's first confirmation. If I recall correctly, both simplewallet and the GUI will have timestamps accompanied to transactions as a new feature. This feature will be present in the upcoming binaries (0.10).