The OSR Sky Portal does not use Excel, but a third-party component to build the reports. This means that a few formulas may not be supported in the third-party component.
Example of a formula that fails in the Portal:
In order for this formula to work, you must change all INDIRECT entries in formulas by adding ;0.
Example of such a formula:
With this formula, it means you are forcing the portal component to interpret it as an R1C1 reference instead of an A1 reference. Excluding the “zero”, it is always interpreted as an A1 reference in the Portal while Excel is a bit smarter and sees that it is actually an R1C1 reference.
With regard to advanced period features, we have a solution guide on this: http://support.onestopreporting.com/support/solutions/articles/1000152763-advanced-period-functions