Contract Start Date Missing inside Contract Information

[philgenb]

In the current design of our Contract Details page, the 'Start Date', which indicates the start of the contract, is not currently stored in the database. If we wish to retain this field, it should be incorporated into the contract information stored in our database. Additionally, for Slack integration, a Slack username should be stored in an appropriate location.

[zyanyna]

Timesheet-Validation rule: A Hiwi should not be able to create a timesheet before the month their contract started.