I have found the issue as you have mentioned. I have logged your issue into our issue tracking system with an issue id: CELLSNET-17776. We will figure out your issue soon.
However if you see the 'Baseline Authorized vs. Forecast' chart then you should find there is different between the original version and converted version.
The most importance part is 'We are here' blue line shift to March and original chart is in Feb.
We almost certain to use the product, please let us know if it can be fixed.
Please try our latest version v5.0.0.3(attached). We could not find the issue you mentioned in the latest version of the Aspose.Cells, please refer to the attachment.
Well, the datetime is rendered because your template Excel file has a footer (datetime) set on the left/first section for the worksheet. For confirmation, please check the MS Excel print preview for the sheet, you can also open the PageSetup dialog box for the worksheet in MS Excel and click on Header/Footer tab of the dialog.
After an initial test, I can find the issue. Your issue has been logged into our issue tracking system with an id: CELLSNET-18925. We will figure it out soon.