Okay, seemingly it only works the other way round.
set the initial value of a parameter to 0 and then:
time>1800 then parameter=new_value
Furthermore, if you want to conditionally compare a quantity different from time one needs to multiply with another parameter equal to one that has the respective unit.
Example: let the unit of X be 'Molarity', then then the event has to look like this:
X>10000*unit_parameter unit_parameter is a globally defined parameter of value 1 with the unit 'Molarity'
