id<-c(rep(1,5),rep(2,5),rep(3,5))
year<-c(rep(2000:2004,3))
event<-c(0,0,1,0,0,0,0,0,0,0,0,1,0,1,0)
sequence<-c(1,1,1,2,2,1,1,1,1,1,1,1,2,2,3)
seq_year<-c(1,2,3,1,2,1:5,1,2,1,2,1)
df<-data.frame(id,year,event,sequence,seq_year)
set.seed(1)
df$x<-rnorm(15)

df
    id year event sequence seq_year          x
1   1 2000     0        1        1 -0.6264538
2   1 2001     0        1        2  0.1836433
3   1 2002     1        1        3 -0.8356286
4   1 2003     0        2        1  1.5952808
5   1 2004     0        2        2  0.3295078
6   2 2000     0        1        1 -0.8204684
7   2 2001     0        1        2  0.4874291
8   2 2002     0        1        3  0.7383247
9   2 2003     0        1        4  0.5757814
10  2 2004     0        1        5 -0.3053884
11  3 2000     0        1        1  1.5117812
12  3 2001     1        1        2  0.3898432
13  3 2002     0        2        1 -0.6212406
14  3 2003     1        2        2 -2.2146999
15  3 2004     0        3        1  1.1249309

summary(coxph(Surv(year-1,year,event) ~ x + cluster(id) + strata(sequence), data=df)) ###tstart from year
summary(coxph(Surv(seq_year-1,seq_year,event)~x + cluster(id) + strata(sequence), data=df)) ###tstart from sequence