I'm not sure what data you're using but I created an example that might be structured as you described - it uses the tidyverse.
library(tidyverse)
df <- tibble(Date = list(as.Date("2001-01-05"),
as.Date("2015-09-05"),
as.Date("2075-12-05")),
var = c(1, 3, 4))
The df looks like this printed out:
> df
# A tibble: 3 x 2
Date var
<list> <dbl>
1 <date [1]> 1
2 <date [1]> 3
3 <date [1]> 4
You could expand the list of dates like so:
df %>% unnest(cols = c(Date))
Date var
<date> <dbl>
1 2001-01-05 1
2 2015-09-05 3
3 2075-12-05 4