With the "Weekly Cal 3" stack the date fields were not large enough to hold the full date. Therefore, the 14th of March would show as 1. This is caused by changes in the HyperCard text engine, where non-proportionally spaced fonts need a bit more room to be displayed. Enlarging the individual fields solves the issue.
The issues with "Monthly Cal" are a bit more complex. issues occurred as a result of some scripting errors encountered when using the "Extend" button.
This article is adapted from the Claris Tech Info database.