How does one include a "linear time trend"

Question

I've seen this mentioned often in papers, but I never fully got what it means. Below I simulating some diff-in-diff type of data with Stata.

How would a "linear trend" be included in a two-way fixed-effects regression?

** make some random data
set seed 35135
clear all
set obs 10000
gen id = ceil(_n/30)
bysort id: gen t = _n
bysort id: gen D = (id>200 & t>15)
gen y = abs(int(rnormal() * 1000))
gen x = floor(abs(rlogistic()) * 15 + abs(rlogistic() * 5))
bysort id (x): replace x = x[_N]

label var id "group"
label var t "time"
label var D "treated"
label var y "outcome"
label var x "control"

** Some DiD regression
reg y i.D#i.t i.t i.id, robust