Is it possible to define a datetime variable in dql?

I want to define a query that will return the max date comparing with a date that I give
I could not find a way to define a datetime variable.

I tried this:

    calculate_date_min_max_dash1(func: uid("0x1")) {
      ud as update_date
      cd as math("2021-07-25T11:32:00Z")
      mx as math (max(ud,cd))
    }  show_max_date (func: uid("0x1")) {

its seems to be impossible to define my own datetime var for comapring.

Nope, not possible.

See this Custom Block - Discussion