t1=interval(a, i)

ff=as.Date(c(b,c))  改为  ff=c(b,c)

官方文档说，
The index seq in a for loop is evaluated at the start of the loop; changing it subsequently does not affect the loop. The variable var has the same type as seq. If seq is a factor (which is not strictly allowed) then its internal codes are used: the effect is that of as.integer not as.vector.
你的i会和ff保持同样的type，而typeof(ff)是double，会将i视为double型，i转化成了数字，导致了NA

感谢！问题已经解决了，谢谢~！

感谢！问题已经解决了，谢谢~！