Details
Description
Noticed this on my production version of 4.2.6 and verified on demo 4.4.3.
When entering a "discounts by sign up date" set in civievent, if you leave the fee blank or enter zero, the fee is not saved, and no warning is issued. Since you can enter a zero amount in a regular fee, you should be able to enter it here.
This appears to be the same as http://issues.civicrm.org/jira/browse/CRM-5436 but that was fixed in 3.0.3.