Time-Dependent Workflow Limitations:

Time triggers don’t support minutes or seconds.
Time triggers can’t reference the following:

• DATE or DATETIME fields containing automatically derived functions, such as TODAY or NOW.
• Formula fields that include related-object merge fields.

You can’t add or remove time triggers if:

• The workflow rule is active.
• The workflow rule is deactivated but has pending actions in the queue.
• The workflow rule evaluation criteria is set to Evaluate the rule when a record is: created, and every time it’s edited.
• The workflow rule is included in a package.