[R] Negative exponential fit

[rch4]

Thank you for your suggestions. We are currently working with an the nls
function, but are having trouble getting a curve that fits our data. We have
tried using the following formula:

mod1 = nls(time~a*exp(b*(date)), start=list(a=4.178,b=-1.18))
av=mdate
bv=predict(mod1,list(mdate=av))
length(av)
length(bv)
lines(av,bv)

We've been trying to work with your suggestions but aren't having any luck
thus far. We've also tried to exponentiate the times in hopes they would
produce a better fit, but can't seem to produce a formula without getting
errors. 

Any further help/explanations would be appreciated. 

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