Working Days Between Two dates in two Tables

This is how you can calculate the number of days elapsed between the start and end dates when they reside in different tables.  Paste the script below into a blank query.  This contains both sample tables with the result added to the sample end dates table.  If you view the query in the advanced editor or view the step script in the formula bar you will see the comments I added for various steps to explain what is going on in the script.

Note - this could further be enhanced to handle situations in which a ticket appears with an end date but not with a start date, if the start date is after the end date, etc. 

SCREENSNIP

SCRIPT

let

    SampleStartDates = Table.TransformColumnTypes(Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUTLWN7LQNzIwMlKK1YlWMgKKmOibQAViAQ==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Ticket = _t, #"Start Date" = _t]), {{"Start Date", type date}}),
    SampleEndDates = Table.TransformColumnTypes(Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUTLRN9E3MjAyUorViVYyAgsYG0BFYgE=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Ticket = _t, #"End Date" = _t]),{{"End Date", type date}}),
    DaysElapsed = Table.AddColumn (SampleEndDates, "Days Elapsed", each 
        Duration.Days (
            // The value of the End Date column from the current table.
            [End Date] - 
            // The value of the Start Date column 
            // for the first row with a matching Ticket, from the other table.
            Table.SelectRows(SampleStartDates, (x)=> x[Ticket]=[Ticket]){0}[Start Date]
        ), Int64.Type 
    )
in
    DaysElapsed

To further develop the previous solution I provided, if you have scenario in which there is a ticket ID with an end date that does not exist in the start date table, without any other error handling the value would return an error, shown below in the Days Elapsed column.  If you wanted to specify a different (number) value to be returned instead of an error, you could implement a try-otherwise statement, which would return the specified number, shown below in the Days Elapsed2 column.

SNIP (Ticket 3 exists in the EndDate table but not in the StartDate table)

SCRIPT

let

    SampleStartDates = Table.TransformColumnTypes(Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUTLWN7LQNzIwMlKK1YlWMgKKmOibQAViAQ==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Ticket = _t, #"Start Date" = _t]), {{"Start Date", type date}}),
    SampleEndDates_AllRecordsMatch = Table.TransformColumnTypes(Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUTLRN9E3MjAyUorViVYyAgsYGyBEjIEipvqGUIFYAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Ticket = _t, #"End Date" = _t]),{{"End Date", type date}}),
    // New table with a record that does not have a start date.
    SampleEndDates_WithExtraRecord = Table.TransformColumnTypes(Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMlTSUTLRN9E3MjAyUorViVYyAgsYGyBEjIEipvqGUIFYAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Ticket = _t, #"End Date" = _t]),{{"End Date", type date}}),
    DaysElapsed_WithoutHandlerForExtraRecord = Table.AddColumn (SampleEndDates_WithExtraRecord, "Days Elapsed", each 
        Duration.Days (
            // The value of the End Date column from the current table.
            [End Date] - 
            // The value of the Start Date column 
            // for the first row with a matching Ticket, from the other table.
            Table.SelectRows(SampleStartDates, (x)=> x[Ticket]=[Ticket]){0}[Start Date]
        ), Int64.Type 
    ),
    DaysElapsed_WithHandlerForExtraRecord1 = Table.AddColumn (DaysElapsed_WithoutHandlerForExtraRecord, "Days Elapsed2", each 
        try Duration.Days (
            // The value of the End Date column from the current table.
            [End Date] - 
            // The value of the Start Date column 
            // for the first row with a matching Ticket, from the other table.
            Table.SelectRows(SampleStartDates, (x)=> x[Ticket]=[Ticket]){0}[Start Date] 
        ) otherwise 0, Int64.Type 
    )
in
    DaysElapsed_WithHandlerForExtraRecord1