Execution time changes when report is downloaded

Well, I noticed something very strange today.

I am calculating the dashboard execution time using the following logic

="Execution Time: " +
IIf(System.DateTime.Now.Subtract(Globals!ExecutionTime).TotalSeconds < 1, "0 seconds", 
(
IIf(System.DateTime.Now.Subtract(Globals!ExecutionTime).Hours > 0, System.DateTime.Now.Subtract(Globals!ExecutionTime).Hours & " hour(s), ", "") +
IIf(System.DateTime.Now.Subtract(Globals!ExecutionTime).Minutes > 0, System.DateTime.Now.Subtract(Globals!ExecutionTime).Minutes & " minute(s), ", "") +
IIf(System.DateTime.Now.Subtract(Globals!ExecutionTime).Seconds > 0, System.DateTime.Now.Subtract(Globals!ExecutionTime).Seconds & " second(s)", ""))
)

But when I download the SSRS report in PDF format unto my PC, its changing the values.

Like 45 secs to 51 secs (just one example). Is there something wrong with my query logic ?

Thanks!!