When the language configured in JIRA is different with the Machine OS, exporting filter to excel will cause the date mismatch in term of format and display.
JIRA should parse the Date format in number(perhaps something) rather than the date as the Excel will use the Regional Language in the Machine.
JIRA's Language = English
Machine OS's Language = Dutch (Belgium)
When exporting Date Custom Field in JIRA which is supposed to be dd/MMM/yy
The Date in Excel should all be consistence but due to some conversion made by Excel based on Regional Language of OS's Language.
Set the same language in JIRA and the OS
Parse the date as number instead of MMM?