When using the API Explorer in the Developer Portal, there is a parameter being added on every call in the form "_=". For example, "https://devportal.company.com/dev-apis/users?$expand=UserGroups&_=1547821843851" What's the reason for this parameter? Do portal published APIs strip out this parameter in the gateway policy before reaching the API?
The parameter at the end of the API request is the UNIX timestamp of the request being made. You can use any online UNIX timestamp converter, such as EpochConverter to convert that parameter into a readable timestamp.