xtset help

Hi there guys,

I'm new to Stata for about a week and I'm really struggling to xtset my data. I currently have approximately 3,400 observations (company data per month). For each company, there is monthly data from 31/01/1990 through to 31/12/2015.

something like this: I have already managed to convert company to numeric type. But then when I go to xtset my data, I keep getting 109 or 111 errors. Am I correct in assuming the problem is due to the months not being properly formatted for Stata?

Many thanks for the help,

Neil.