Conversion will be based on when the vendor CLAIMS the amount from your bank. That day's rates + a 3.5% mark up charge (conversion charge) would be what would show on your statement. Some banks optionally add a 12.36% tax on that 3.5% fee. Some include it in the 3.5%. HDFC is in the former category.
Cloud-S, Standard Chartered told me that they charge. It is another thing that I have hardly ever been able to match their rate against any site / reference ever. I really like HDFC's transparency in this regard. Both HDFC and ICICI use the same reference rate. Standard Chartered, I doubt (though they claim they do)