Subtract days power query BI

I believe you want Date.AddDays([Last INV Date], -10)

Here is an example query I created:

let
Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUTLUN9Q3MjAyUIrViVYyAAoYIQsYAQWM4QKxAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [Item = _t, #"Last INV Date" = _t]),
#"Changed Type" = Table.TransformColumnTypes(Source,{{"Item", type text}, {"Last INV Date", type date}}),
#"Added Custom" = Table.AddColumn(#"Changed Type", "Custom", each if [Item] = "0" then Date.AddDays([Last INV Date],-10) else [Last INV Date])
in
#"Added Custom"

Hi bhelou 

Try this.

if [Day 2]="0" then Date.AddDays(  [Last INV Date], -10 ) else [Last INV Date]

Hello bhelou 

the value is a number, non a text. so use this formula

if [Day 2]=0 then Date.AddDays( [Last INV Date], -10 ) else [Last INV Date]

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Kudoes are nice too

Have fun

Jimmy