Slope of logisitic function at a certain point (p=0.5)

Can anybody help me how to calculate the slope of a logistic function at a certain point (p=0.5) using Matlab?

The function looks as follows:

log(p/1-p) = intercept + slope x t

data =

   56.2500         0    5.0000
   78.7500         0    5.0000
  101.2500    3.0000    5.0000
  123.7500    4.0000    5.0000
  146.2500    5.0000    5.0000
  168.7500    4.0000    5.0000
  191.2500    5.0000    5.0000
  213.7500    5.0000    5.0000

[b1,d1,s1] = glmfit(data(:,1),[data(:,2) data(:,3)],'binomial')

b1 =

   -6.4717  --- Intercept
    0.0601  --- slope

How to calculate the slope at p=0.5 ? Any suggestions?

Thanks, Mark