Differential equation codes?

Hi There,
I like to define a differential equation as dDVS/dt = VR, which shows
daily (t is one day)increment of DVS from its initial value of zero by
change rate of VR and it stops its calculation when DVS is 2.0
It can be also defined in another way as:
DVS = Integration (0., VR)

First: I wonder how to define this in Pascal?
second: Is there any library file which I can find and use it?
Thanks for your help
E-mail: sbx...@sbn2.phes.nottingham.ac.uk