Here is a query you can use to create a calendar. Link the Date field from your table to the Month field from the calendar. This should help with your calculation.
= (StartDate as date, EndDate as date, optional Culture as nullable text) as table =>
let
DayCount = Duration.Days(Duration.From(EndDate - StartDate)),
Source = List.Dates(StartDate,DayCount,#duration(1,0,0,0)),
TableFromList = Table.FromList(Source, Splitter.SplitByNothing()),
ChangedType = Table.TransformColumnTypes(TableFromList,{{"Column1", type date}}),
RenamedColumns = Table.RenameColumns(ChangedType,{{"Column1", "Date"}}),
InsertYear = Table.AddColumn(RenamedColumns, "Year", each Date.Year([Date])),
InsertQuarter = Table.AddColumn(InsertYear, "QuarterOfYear", each Date.QuarterOfYear([Date])),
InsertMonth = Table.AddColumn(InsertQuarter, "MonthOfYear", each Date.Month([Date])),
InsertFinYearNumber = Table.AddColumn(InsertMonth, "FinYearNumber",each if [MonthOfYear] >= 7 then [Year] else [Year] -1 ),
InsertFinYearDisplay = Table.AddColumn(InsertFinYearNumber, "FinYear" ,each "FY" & Text.End(Number.ToText([FinYearNumber],"D",""),2) & "/" & Text.End(Number.ToText([FinYearNumber]+1,"D",""),2)),
InsertFinMonth = Table.AddColumn(InsertFinYearDisplay, "FinMonth", each if [MonthOfYear] >= 7 then [MonthOfYear] - 6 else [MonthOfYear] + 6 ),
InsertFinQuarterNumber = Table.AddColumn(InsertFinMonth, "FinQuarterNumber", each if [QuarterOfYear] > 2 then [QuarterOfYear] -2 else [QuarterOfYear] + 2),
InsertFinQuarterDisplay = Table.AddColumn(InsertFinQuarterNumber ,"FinQuarter", each "FQ" & Number.ToText([FinQuarterNumber],"D","") ),
InsertDay = Table.AddColumn(InsertFinQuarterDisplay , "DayOfMonth", each Date.Day([Date])),
InsertDayInt = Table.AddColumn(InsertDay, "DateInt", each [Year] * 10000 + [MonthOfYear] * 100 + [DayOfMonth]),
InsertMonthName = Table.AddColumn(InsertDayInt, "MonthName", each Date.ToText([Date], "MMMM", Culture), type text),
InsertCalendarMonth = Table.AddColumn(InsertMonthName, "MonthInCalendar", each (try(Text.Range([MonthName],0,3)) otherwise [MonthName]) & " " & Number.ToText([Year])),
InsertCalendarQtr = Table.AddColumn(InsertCalendarMonth, "QuarterInCalendar", each "Q" & Number.ToText([QuarterOfYear]) & " " & Number.ToText([Year])),
InsertDayWeek = Table.AddColumn(InsertCalendarQtr, "DayInWeek", each Date.DayOfWeek([Date],1)+1),
InsertDayName = Table.AddColumn(InsertDayWeek, "DayOfWeekName", each Date.ToText([Date], "dddd", Culture), type text),
InsertWeekEnding = Table.AddColumn(InsertDayName, "WeekEndingFriday", each Date.EndOfWeek([Date],6), type date),
InsertCurrentThursday = Table.AddColumn(InsertWeekEnding, "CurrentThursday", each Date.AddDays([Date], -Date.DayOfWeek([Date],1) + 3), type date),
InsertISOWeekJan4 = Table.AddColumn(InsertCurrentThursday, "ISOWeekJan4", each Date.FromText(Number.ToText(Date.Year([CurrentThursday])) & "-01-04") ,type date),
InsertISOWeekYear = Table.AddColumn(InsertISOWeekJan4, "ISOWeekYear", each Date.Year([CurrentThursday])) ,
InsertISOWeekFirstMon = Table.AddColumn(InsertISOWeekYear, "ISOWeekFirstMon", each
if [CurrentThursday] < [ISOWeekJan4]
then Date.AddDays([CurrentThursday],-3)
else Date.AddDays([ISOWeekJan4], - Date.DayOfWeek([ISOWeekJan4],1) )
,type date),
InsertISOWeekNum = Table.AddColumn(InsertISOWeekFirstMon, "ISOWeekNum", each Number.RoundUp(((Duration.Days(Duration.From([Date] - [ISOWeekFirstMon]))+1) /7 )), type number),
InsertISOWeekID = Table.AddColumn(InsertISOWeekNum, "ISOWeekID", each [ISOWeekYear] * 100 + [ISOWeekNum], type number),
InsertISOWeekName = Table.AddColumn(InsertISOWeekID, "ISOWeekName", each Text.From([ISOWeekYear]) & "W" & Text.End( "0" & Text.From(([ISOWeekNum]*10) + [DayInWeek]),3)),
InsertISOWeekNameLong = Table.AddColumn(InsertISOWeekName, "ISOWeekNameLong", each Text.From([ISOWeekYear]) & "-W" & Text.End( "0" & Text.From([ISOWeekNum]),2) & "-" & Text.From([DayInWeek])),
fnPeriod445a = (weekNum) => let
Periods =
{
{(x)=>x<5, [P=1,Q=1]},
{(x)=>x<9, [P=2,Q=1]},
{(x)=>x<14, [P=3,Q=1]},
{(x)=>x<18, [P=4,Q=2]},
{(x)=>x<22, [P=5,Q=2]},
{(x)=>x<27, [P=6,Q=2]},
{(x)=>x<31, [P=7,Q=3]},
{(x)=>x<35, [P=8,Q=3]},
{(x)=>x<40, [P=9,Q=3]},
{(x)=>x<44, [P=10,Q=4]},
{(x)=>x<48, [P=11,Q=4]},
{(x)=>true, [P=12,Q=4]}
},
Result = List.First(List.Select(Periods, each _{0}(weekNum))){1}
in
Result,
InsertPeriod445 = Table.AddColumn(InsertISOWeekNameLong, "Period445Record", each fnPeriod445a([ISOWeekNum])),
ExpandPeriod445 = Table.ExpandRecordColumn(InsertPeriod445, "Period445Record", {"P","Q" }, {"Period445", "Quarter445"}),
RemovedColumns = Table.RemoveColumns(ExpandPeriod445,{"CurrentThursday", "ISOWeekFirstMon"})
in
RemovedColumns
