Period(start()) is the way to go as others have said, but also change the summary to First Non Blank.
First, make sure Total of All is turned on in your Time Range.
Create a SYS Time Filter module using the Time Range, in my case FY18. Create a line item with Period(start()) with a format of Time Period (month) if you are using month. Next, change the summary to First non-blank.
for a result of:
Now, you can get the final solution in a line item using a Select statement which is ok because it is structural data.