period_example

## Using as.Date with an arbitrary year you can define each peirod
pre.emg.period=seq(as.Date("2021-03-01"),as.Date("2021-03-20"),"1 days")
emg.period=seq(as.Date("2021-03-21"),as.Date("2021-04-14"),"1 days")
mate.period=seq(as.Date("2021-04-15"),as.Date("2021-05-15"),"1 days")
gest.period=seq(as.Date("2021-05-16"),as.Date("2021-08-23"),"1 days")
post.part.period=seq(as.Date("2021-08-24"),as.Date("2021-10-31"),"1 days")

# fake data
dat=data.frame(Date=seq(as.Date("2019-01-01"),as.Date("2022-12-31"),"1 days"))
dat$Yr=as.numeric(format(dat$Date,'%Y'))

## A nested ifelse statement combined with different periods will delineate your periods
dat$period=with(dat,ifelse(format(Date,"%m-%d")%in%format(pre.emg.period,"%m-%d"),"pre.emergence",
                           ifelse(format(Date,"%m-%d")%in%format(emg.period,"%m-%d"),"emergence",
                                  ifelse(format(Date,"%m-%d")%in%format(mate.period,"%m-%d"),"mating",
                                         ifelse(format(Date,"%m-%d")%in%format(gest.period,"%m-%d"),"gestation",
                                                ifelse(format(Date,"%m-%d")%in%format(post.part.period,"%m-%d"),"post.partum",NA
                                                       ))))))

## Sanity check
library(plyr)
ddply(dat,c("Yr","period"),summarise,min.date=min(Date),max.date=max(Date))