Just as a note a side, you can just do {year, month, day} = date, no need to convert the tuple to a list first. Pattern matching is not limited to lists.

ah, you’re being tripped by a subtlety of pattern matching on lists, remember that a linked list is a list of lists, so tail is always a list, possibly empty (well, except for the empty list which has no tail)–here, check out this:

iex(1)> [date|val] = [1,2]
[1, 2]
iex(2)> date
1
iex(3)> val
[2]
iex(4)> [date|[val]] = [1,2]
[1, 2]
iex(5)> date
1
iex(6)> val
2
# the way you really want to do it:
iex(7)> [date,val] = [1,2]
[1, 2]
iex(8)> date
1
iex(9)> val
2

Also

dat = Tuple.to_list(date)
[year, month, day] = dat

should just be {year, month, day} = date

and pretty sure you need Integer.to_string(val)

[EDIT] and I see you’re already calling Integer.to_string–it was just “below the fold” of your message when I was composing this