DATEFIRST

There are 3 functions.
For instructions on how to add a function to a workbook refer to the page under Inserting Functions

DATEFIRST_INTHISWEEK

Returns the date of the first day of a week.

Public Function DATEFIRST_INTHISWEEK( _ 
Optional ByVal dtDateValue As Date) As Long 

    Application.Volatile 
    
    DATEFIRST_INTHISWEEK = dtDateValue - VBA.Weekday(dtDateValue, vbUseSystemDayOfWeek) + 1 
End Function 

DATEFIRST_INTHISMONTH

Returns the date of the first day of a month.

Public Function DATEFIRST_INTHISMONTH( _ 
Optional ByVal dtDateValue As Date) As Long 

    Application.Volatile 
    If VBA.IsMissing(dtDateValue) Then 
        dtDateValue = VBA.Date 
    End If 

    DATEFIRST_INTHISMONTH = VBA.DateSerial(VBA.Year(dtDateValue), VBA.Month(dtDateValue), 1) 
End Function 

DATEFIRST_INTHISYEAR

Returns the date of the first day of a year.

Public Function DATEFIRST_INTHISYEAR( _ 
Optional ByVal dtDateValue As Date) As Long 

    Application.Volatile 
    
    If VBA.IsMissing(dtDateValue) Then 
        dtDateValue = VBA.Date 
    End If 
    
    DATEFIRST_INTHISYEAR = VBA.DateSerial(VBA.Year(dtDateValue), 1, 1) 
End Function