在MatchIt中没有内置的方法来做到这一点，但是matchit()允许你提供任何你想要的距离矩阵，禁止的匹配用Inf标记。所以我们可以做的是创建一个距离矩阵，然后对于每个暴露的单元，如果处理单元的time小于或等于控制单元的time，则将到每个控制单元的距离设置为Inf。

# Estimate a propensity score
df$ps <- glm(exposure ~ sex * age * time, data = df,
          family = binomial)$fitted

# Create distance matrix of ps, with treated units
# on rows and control units on columns
ps_dist <- MatchIt::euclidean_dist(exposure ~ ps, data = df)

# Set to `Inf` any forbidden matches
exposed_times <- df$time[[df$exposure == 1]]
unexposed_times <- df$time[[df$exposure == 0]]
ps_dist[outer(exposed_times, unexposed_times, "<=")] <- Inf

# Supply the distance matrix to `matchit()`
m <- MatchIt::matchit(exposure ~ sex + age + time, data = df,
                      distance = ps_dist)