timesteps are stored as colnames(Y), but column names cannot be Date objects. Instead, we use strings. If they are strings convertible to dates, return that
Examples
# \donttest{
data(california_prop99)
setup <- panel.matrices(california_prop99)
timesteps(setup$Y)
#> [1] "1970" "1971" "1972" "1973" "1974" "1975" "1976" "1977" "1978" "1979"
#> [11] "1980" "1981" "1982" "1983" "1984" "1985" "1986" "1987" "1988" "1989"
#> [21] "1990" "1991" "1992" "1993" "1994" "1995" "1996" "1997" "1998" "1999"
#> [31] "2000"
# }