Fixed date-format for 'date-at' parameters

The 'date-at' parameter, unlike all the other date parameters strictly requires the YYYY-M-D format (see http://mite.yo.lk/api/zeiten.html for details). In the current implementation it screws up months and days.
I fixed all occurrences of 'date-at' (there are only two ;-), and also extended the XML deserialization test to check for correct date deserialization.