hledger-interest is a small command-line utility based on Simon Michael's hlegder library. Its purpose is to compute interest for a given ledger account. Using command line flags, the program can be configured to use various day counting conventions, such as "act/act", "30/360", "30E/360", and "30/360isda". Furthermore, it supports several of different interest schemes, i.e. annual interest with a fixed rate and the scheme mandated by the German law § 288 BGB Verzugszinsen. Extending support for other schemes is fairly easy, but currently requires hacking the source code.
An overview over the available run-time options can be displayed by
running "hleder-interest --help":
Usage: hledger-interest [OPTION...] ACCOUNT
-v --verbose echo input ledger to stdout (default)
-q --quiet don't echo input ledger to stdout
--today compute interest up until today
-f FILE --file=FILE input ledger file (pass '-' for stdin)
-s ACCOUNT --source=ACCOUNT interest source account
-t ACCOUNT --target=ACCOUNT interest target account
--act use 'act' day counting convention
--30-360 use '30/360' day counting convention
--30E-360 use '30E/360' day counting convention
--30E-360isda use '30E/360isda' day counting convention
--constant=RATE constant interest rate
--annual=RATE annual interest rate
--annual-schedule=SCHEDULE schedule of annual interest rates.
syntax: '[(Date1,Rate1),(Date2,Rate2),...]'
--bgb288 compute interest according to German BGB288
When run, hledger-interest reads the ledger
file designated by the
--file flag and filters all transactions that change the account
specified on the command line. All other accounts will be ignored. Every
time a transaction modifies the given account's balance -- thereby
changing the amount of money that earns interest --, hledger-interest
transfers the interest that accrued so far. Interest will be debited
from the account designed by the --source flag and credited to the
account designed by the --target flag.
Suppose that you've loaned 1000 Euro from your bank at an annual
interest rate of 5%, and that you would like to see how interest
develops over time. Then you would create a ledger file, say
loan.ledger, that looks something like this:
2010/09/26 Loan
Assets:Bank EUR 1000.00
Liabilities:Loan
Now, ledger-interest is run to determine the interest up until today:
$ hledger-interest -f loan.ledger --act --annual=0.05 --today -s Expenses:Interest -t Liabilities:Loan:Interest Liabilities:Loan
2010/09/26 Loan
Assets:Bank EUR 1000.00
Liabilities:Loan
2010/12/31 5.00% interest for EUR -1000.00 over 96 days
Liabilities:Loan:Interest EUR -13.15
Expenses:Interest
2011/08/22 5.00% interest for EUR -1000.00 over 234 days
Liabilities:Loan:Interest EUR -32.05
Expenses:Interest
Note a separate credit account for the interest was chosen:
Liabilities:Loan:Interest. Consequently, interest accrued in one
interest period does not earn interest in the following periods. If
interest is credited to the main account instead, that behavior changes:
$ hledger-interest -f loan.ledger --act --annual=0.05 --today -s Expenses:Interest -t Liabilities:Loan Liabilities:Loan
2010/09/26 Loan
Assets:Bank EUR 1000.00
Liabilities:Loan
2010/12/31 5.00% interest for EUR -1000.00 over 96 days
Liabilities:Loan EUR -13.15
Expenses:Interest
2011/08/22 5.00% interest for EUR -1013.15 over 234 days
Liabilities:Loan EUR -32.48
Expenses:Interest
Of course, loans are supposed to be paid back, and these payments change
the amount of interest accrued. Suppose that load.ledger would be
extended by the following transactions:
2010/12/11 Payment
Assets:Bank EUR -150.00
Liabilities:Loan
2011/03/07 Payment
Assets:Bank EUR -300.00
Liabilities:Loan
2011/08/21 Payment
Assets:Bank EUR -150.00
Liabilities:Loan
Then interest would develop as follows:
$ hledger-interest -f loan.ledger --act --annual=0.05 -s Expenses:Interest -t Liabilities:Loan Liabilities:Loan
2010/09/26 Loan
Assets:Bank EUR 1000.00
Liabilities:Loan
2010/12/11 5.00% interest for EUR -1000.00 over 76 days
Liabilities:Loan EUR -10.41
Expenses:Interest
2010/12/11 Payment
Assets:Bank EUR -150.00
Liabilities:Loan
2010/12/31 5.00% interest for EUR -860.41 over 20 days
Liabilities:Loan EUR -2.36
Expenses:Interest
2011/03/07 5.00% interest for EUR -862.77 over 66 days
Liabilities:Loan EUR -7.80
Expenses:Interest
2011/03/07 Payment
Assets:Bank EUR -300.00
Liabilities:Loan
2011/08/21 5.00% interest for EUR -570.57 over 167 days
Liabilities:Loan EUR -13.05
Expenses:Interest
2011/08/21 Payment
Assets:Bank EUR -150.00
Liabilities:Loan
Last but not least, there is a special case known as "Verzugszinsen" in
German law, which applies when someone is supposed to pay a bill, but
fails to do so on time. For every day past the deadline, interest
accrues according to terms specified in § 247
BGB. The command line
flag --bgb288 enables this scheme in hledger-interest.
Let's assume that customer ACME is supposed to pay 35 Euro by 2010/09/15, but the money actually arrives almost half a year late:
2010/09/15 Services rendered to Customer ACME
ACME 1 hour @ EUR 35.00
Receivable:ACME
2011/03/17 ACME
ACME EUR 35.00
Receivable:ACME
According to German law, you are entitled to the following interest:
$ hledger-interest -f acme.ledger --quiet --bgb288 -s Income:Interest -t Receivable:ACME:Interest Receivable:ACME
2010/12/31 5.12% interest for EUR 35.00 over 107 days
Receivable:ACME:Interest EUR 0.53
Income:Interest
2011/03/17 5.12% interest for EUR 35.00 over 76 days
Receivable:ACME:Interest EUR 0.37
Income:Interest
So, if you're smart, then you'll book the payment so that the accrued interest is paid first:
2011/03/17 ACME
ACME EUR 35.00
Receivable:ACME:Interest EUR -0.90
Receivable:ACME
This gives the following transaction history for the ACME account:
$ hledger-interest -f acme.ledger --bgb288 -s Income:Interest -t Receivable:ACME:Interest Receivable:ACME |
hledger -f - reg Receivable:ACME
2010/09/15 Services rendered .. Receivable:ACME EUR 35.00 EUR 35.00
2010/12/31 5.12% interest for.. Re:ACME:Interest EUR 0.53 EUR 35.53
2011/03/17 5.12% interest for.. Re:ACME:Interest EUR 0.37 EUR 35.90
2011/03/17 ACME Re:ACME:Interest EUR -0.90 EUR 35.00
Receivable:ACME EUR -34.10 EUR 0.90