I imported my monthly time series data from 1824-1940 from excel to Stata. The time variable is in string format because my Excel does not accept the year before 1900.

The data in excel looks like:
Year price
01/1824 2,4
05/1825 2,7
06/1825 2,3

I converted the string to datetime in Stata by the command:

gen t=date(year, "MY")

I try to draw the lineplot of the price over t but Stata keeps reporting "time variable not set, use -tsset varname...-".
However, when I input the command
tsset t
Stata tells me that "repeated time values in sample"

Is it because my data is too old? Does anyone know how to fix it?

Thank you