Arcade Date expression for symbology in ArcGIS Pro 2.2.0

[BruceAnderson1]

Arcade Date expression for symbology in ArcGIS Pro 2.2.0

I'm trying to use an Arcade expression to symbolize data based on a date field. I want to display records for each Year/Month combination. the expression verifies and seems to work but if I look at the counts for the year/month groupings they are off by one month. e.g. the number of records for 2018/5 are showing as 2018/4. Any idea what might be causing this? 

here is the expression that I'm using: 

Year($feature.observation_date) + "/" + Month($feature.observation_date)

[KenBuja · Accepted solution]

From the documentation: Date Functions | ArcGIS for Developers 

Returns the month of the given date. Values range from 0-11 where January is 0 and December is 11.

[BruceAnderson1]

Thank you.