SSRS time of datetime parameter is not refreshing when Report autofresresh runs (every 30 seconds)

Question

I have two parameters in my report (SSRS). First parameter: Yesterday date+time Second parameter: Now date+time I use the follow "expressions" commands to get those parameters when I run my report. To get yesterday date+time:

=DateAdd("d",-1,Now())

and to get now date+time:

=Now()

I have set Autorefresh property to 30 seconds. and when I run the report loads fine and after 30 seconds I see it to do something like refreshing but the datetime parameters on the top of the page keeps freezed so the values in my report are not changing. How can I fix it? Thank you in advance.