STARTOFYEAR Returns the first date of the year in the current context for the specified column of dates.
Syntax
STARTOFYEAR(dates)
Parameters
Dates – A column that contains dates.
YearEndDate – (Optional) A year end date value.
Return Value
A table containing a single column and single row with a date value.
Reference Data
https://docs.microsoft.com/en-us/dax/dax-function-reference