updating date time field with currentDatetime value on change of Status

Hi experts,

I have two fields in one of my custom entities 

1) Request Step(option set)

2)Closed On Date(Datetime field)

when user set request step option value to Closed i have to set the ClosedOnDate field value with current DateTime .

Which approach is better among business rule ,workflow and javascript 

I tried with business rule but there is no current Datetime value available here.

Thanks,

Nagaraju