Ant includes a Tstamp task that allows a property to be set to the current date and time in a given format. Unfortunately there is no way to get that value as a Unix time (epoch).

I have built my own custom invoicing tool using XML, XSLT , XSL-FO and Ant. It is probably a little bit overkill but it is certainly better than being locked into some proprietary Windows application. It also give me more flexibility compared to most online invoicing applications.

One important feature that was missing so far was the scheduling of reminders to check payments. This has been solved thanks to Remember The Milk and some Ant script.