A code that looks numeric is not necessarily a number for calculation. Excel's automatic conversions can remove leading zeros; supported versions expose conversion controls. For identifiers, decide the data type before accepting the import rather than fixing the appearance afterwards.
Use a tiny sample
Create a harmless test CSV with codes 00125, 01002 and 10020. Keep a separate untouched copy. In your spreadsheet's text import workflow, inspect the preview and choose a text type for the code column before loading it. The exact controls depend on the Excel version and import method.
Compare the resulting cells with the original text. Saving a workbook that merely displays five digits is not the same as preserving the original identifier. Reopen an exported test file in a text editor to see what another system would actually receive.
Dates need their own decision
A value such as 03/04/2026 is ambiguous without a stated convention. Ask which order the source uses before turning the column into dates. Do not let a plausible local display settle that question for you.
If the original zeros or digits have already been lost, return to the source file. Do not reconstruct them by guessing. Keep codes as codes, dates as deliberately interpreted dates, and the raw import available until the comparison is complete.