hledger-interest

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.

Examples

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