Easiest would be to make new data frame for regions that should be annotated.
df<-data.frame(xmin=as.Date(c('1970-01-01','1990-01-01')),
xmax=as.Date(c('1980-01-01','2000-01-01')),
ymin=c(-Inf,-Inf),
ymax=c(Inf,Inf),
years=c("1970s","1990s"))
Then use geom_rect()
and this new data frame to add those regions. Legend will be made automatically. With scale_fill_manual() you can change colors.
ggplot(data=economics, mapping=aes(x=date, y=unemploy)) +
geom_line(size=1)+
geom_rect(data=df,aes(xmin=xmin,ymin=ymin,xmax=xmax,ymax=ymax,fill=years),
alpha=0.2,inherit.aes=FALSE)+
scale_fill_manual(values=c("red","green"))