set up months to a time serie

Dear all,

I am trying to set a sequence of  48000 months to a simulation between
10200 - 6201  before present.

However I always get the following message
Error in charToDate(TS) : #character string is not in a standard
unambiguous format
Could anyone help me?
####
library(zoo)
TS1<-brick("TS1.nc")
# dimensions  : 12, 12, 144, 48000  (nrow, ncol, ncell, nlayers)
TS1 <- as.yearmon(10200 - seq(0, 48000)/12)

####

Moreover, I would like to detect changes inside my rasterbrick serie, I
read that package bfast can detect changes in such of time series and
files, anyone experienced with bfast could help me with it as well?

thank you all,

Best

Jackson