Distinguish in date notation depending on project status (planned/started)

Hi, I'm building a dashboard to do some project management. I'm looking to distinguish date notation in a table on my dashboard.

Here's an example of what I'm looking for:

In my source table, I have put estimates of planned start dates in the dd/mm/yy format. However, it would be better to display them as weeks, as they are not as precise. 

All projects I have started on, have a specific start date. Those I'd like to display with their specidic dates. Therefore the column of the dashboard would have a mix of data types. How do I do this?

I'm thinking of creating a table that distinguishes between 'planned' and 'started' projects in a binary way. After that, I'd have to make an if statement somewhere:

If [status] is [planned], then [date notation] is [rounded to weeks]

Elif [status] is [started], then [date notation] is [specific to the day]

But how and where do I do this? I'm rather new to PowerBI.

Thanks in advance!