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Date.AddMonths shifts a date, datetime, or datetimezone value forward or backward by a number of months. It does not consolidate ledger tables. To combine monthly general ledger (GL) exports in Power Query, you use a folder combine or an Append operation, and you can use Date.AddMonths inside that workflow to derive period dates. This article explains each piece, the order to build them in, and the checks you should run before trusting the combined output.
What Date.AddMonths does
Microsoft documents the syntax as Date.AddMonths(dateTime as any, numberOfMonths as number) as any. It accepts a date, datetime, or datetimezone value, and it returns a value of the same type. The Date.AddMonths reference on Microsoft Learn gives these examples:
Date.AddMonths(#date(2011, 5, 14), 5)
// #date(2011, 10, 14)
Date.AddMonths(#datetime(2011, 5, 14, 8, 15, 22), 18)
// #datetime(2012, 11, 14, 8, 15, 22)
The first example moves May 14 forward five months to October 14. The second moves a datetime forward 18 months and keeps the time of day unchanged. A negative month count moves the value backward.
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- It does not read files, merge tables, or remove duplicates.
- It does not define a fiscal calendar, an accounting period, or a period-close date. If your organization’s period runs from the 26th to the 25th, or uses a 4-4-5 calendar, you must write that rule yourself.
- It does not confirm that a ledger export is complete or correct.
Treat Date.AddMonths as a tool for labelling rows with a period, not as the step that produces the consolidated ledger.
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Choose the consolidation method first
Power Query offers three practical routes for combining monthly files or tables. Pick the one that matches where your exports live and how consistent their layouts are.
| Approach | Use it when | How columns are matched | Source documentation |
|---|---|---|---|
| Folder combine (Combine and Transform) | Monthly files land in one folder and share the same format and structure | Power Query applies the transformations from a sample file to every file in the filtered list | Folder connector; Combine files overview |
| Append queries | Each month is already a separate query in the workbook or model | Matched by column header names, not position; unmatched columns produce nulls | Append queries |
| Table.Combine in M | You assemble a list of tables in M code | Appends the tables in the list it receives | Table.Combine |
Folder combine suits a recurring drop folder, because a refresh picks up new files that match your filter. Append suits a fixed set of month queries. Neither approach is universally better; the deciding factors are source organization, schema consistency, and whether each file needs its own cleanup before it joins the others.
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Build a repeatable folder workflow
Keep each month’s export in one dedicated folder, with a consistent file type and column layout. Then follow these steps.
- Connect to the folder. In Power BI Desktop, select Home > Get Data > Folder. In Excel for Windows, select Data > Get Data > From File > From Folder. Enter the folder path and select OK.
- Filter the file list. In the preview, filter the
Extensioncolumn to the file type you expect, and filterFolder PathorNameso that only the intended month’s files remain. Temporary files, archives, and files from other periods should be excluded here, before combining. - Start the combine. Select Combine > Combine and Transform Data. Power Query asks you to choose a sample file and opens a Transform Sample File query.
- Put the per-file cleanup in the sample query. Promote headers, remove blank rows, set data types, and rename columns here. Power Query generates a helper function from these steps and applies it to every file in the filtered list.
- Add lineage columns. Keep the source file name and the accounting period in the output so that every row can be traced to an input file. Use the
Source.Namecolumn that the folder connector provides, and add a period column as described below. - Close and load. Select Home > Close & Load. To pick up next month’s file, add it to the folder and select Refresh.
The combine step assumes every file has the same structure. A file with an extra column, a renamed header, or a different sheet name will not fail cleanly; it will create nulls or misaligned fields downstream. Check the first few files before you rely on the output.
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Derive the period with Date.AddMonths
A common pattern is to store each file’s period start as a date and derive the following month when you need it, for example for an opening-to-closing comparison. Add a custom column with Add Column > Custom Column and use an expression such as:
Date.AddMonths([PeriodStart], 1)If PeriodStart is a date column, the result is a date. If it is text, convert it first with a type conversion step. Test the expression on month-end dates and on a February in a leap year in your own query, because the result for those dates is what your reporting will use. The documentation shows the arithmetic but does not decide which day your reporting period should use.
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Append queries that already exist
If each month is already a separate query, select Home > Append Queries, choose the first table, and add the others. Use Append Queries as New when you want to keep the individual month queries unchanged for audit purposes.
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let
Source = Table.Combine({Jan_GL, Feb_GL, Mar_GL})
in
SourceBefore appending, compare the header names and data types of the month queries. Because matching uses names, a header that reads Acct No in one month and Account in another will produce two columns, each partly empty.
Validation before the output is used
Power Query’s documentation describes how the mechanics work. It does not describe accounting controls, so the checks below are prudent steps for you to adopt according to your organization’s accounting policy, not requirements that Microsoft sets.
- Record counts: compare the row count of each source file, or each month query, with the row count of its rows in the combined output.
- Coverage: confirm that every expected month and account range appears, and that no period appears twice unless you expect it.
- Totals: compare the sum of a control column against the total in the source system’s export for the same period.
- Debits and credits: where your ledger balances, check that debits equal credits for each period in the output.
- Sign, currency, and approvals: confirm the sign convention, currency fields, and any approval steps against your own accounting procedures.
Troubleshooting common problems
- Files from another period appear in the output. The folder filter is too broad. Filter on the period in the file name or in the
Source.Namecolumn. - Columns are full of nulls after an append. A header differs between months. Rename the header in the source query so that the names match exactly.
- A new file does not appear after refresh. The file is outside the folder path or fails your filter. Check the filter on
ExtensionandName. - The date result is one day off or lands on an unexpected day. The input column is text or a datetime with a time component. Convert it to
datebefore callingDate.AddMonths, and test the result on month-end examples.
Power Query refreshes the query you built; it does not schedule itself. For regular unattended refresh, use the refresh options of the tool you publish to, and confirm them with your IT or BI team.
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