@Dariocg
Yes, what you refer to as solar days is more commonly called calendar days or elapsed days in Project terminology.
Depending on what the "design approval" item determines how it is best handled in calendar days. If "design approval" is an actual separate task (i.e. has resources assigned), then create a custom calendar that has 7 working days and set it as the calendar for the task via the Task Calendar field. If the duration of that task is 30 calendar days, set the Duration field to 30ed. Note that if a resource is assigned and that resource's Base Calendar is the Standard 5/8 (or 5/7 if you have a custom Project calendar with a 7 hour workday), you will need to set the option, "scheduling ignores resource calendar" on the Advanced tab of the Task Information window. If the resource's Base Calendar is set to the custom 7 day calendar, then you don't need to set the option to ignore.
If however, the "design approval" is simply a waiting period between two tasks, the approval waiting period can be expressed as a dependency with a lag. For example, let's say the design task (ID 1) is complete and the design approval is a required 30d process before the next task (ID 2) can start. Set a predecessor of 1FS+30ed on task ID 2. That means there will be a delay of 30 calendar (elapsed) days after task ID 1 is finished until task ID 2 can start.
Hope this helps.
John
