Useful extensions to the standard Python datetime features
The dateutil module provides powerful extensions to the standard date time module, available in Python. dateutil can be installed from PyPI using pip (note that the package name is different from the importable name).
JS library for working with recurrence rules for calendar dates
JavaScript library for working with recurrence rules for calendar dates as defined in the iCalendar RFC and more. rrule.js supports recurrence rules as defined in the iCalendar RFC, with a few important differences. It is a partial port of the rrule module from the excellent python-dateutil library. On top of that, it supports parsing and serialization of recurrence rules from and to natural language.
JPotpourri offers some basic java classes to make a programmer's day easier. It includes gui enhancements (widgets, handler) and a lot of easy-to-use utility classes, among them: FileUtil, ZipUtil, DateUtil, CollectionUtil, ...