Understanding Time Series Horizontal Merge Questions

There are two CSV files: area1.csvarea2.csv
There is no item name in the time column, so
If I want to combine horizontally based on time, how can I modify it?
I would appreciate it if you could let me know!
df1=pd.read_csv(area1.csv,index_col=0)
df2=pd.read_csv(area2.csv,index_col=0)
df3=pd.merge([df1,df2], how="outer")
df3.to_csv(area3.csv,index=False,encoding='utf-8')