Writing to #timestamp in LogStash - elasticsearch

Question

I need to write the value of a UNIX timestamp field to #timestamp so that I can correctly index data flowing through logstash, I have this part working. However I also have the requirement that #timestamp's value should be the insertion time. To this end I have made a temporary field that holds #timestamps original value.
Here is what I am working with:
filter {
csv {
separator => " " # <- this white space is actually a tab, don't change it, it's already perfect
skip_empty_columns => true
columns => ["timestamp", ...]
}
# works just fine
mutate {
add_field => {
"tmp" => "%{#timestamp}"
}
}
# works just fine
date {
match => ["timestamp", "UNIX"]
target => "#timestamp"
}
# this works too
mutate {
add_field => {
"[#metadata][indexDate]" => "%{+YYYY-MM-dd}"
}
}
# #timestamp is not being set back to its original value
date {
match => ["tmp", "UNIX"]
target => "#timestamp"
}
# works just fine
mutate {
remove_field => ["tmp"]
}
}
output {
elasticsearch {
hosts => "elasticsearch:9200"
# this works
index => "indexname-%{[#metadata][indexDate]}"
}
}
The Problem is here:
date {
match => ["tmp", "UNIX"]
target => "#timestamp"
}
#timestamp is not being set back to its original value. When I check the data it has the same value as the timestamp field.

Answer 1

When you add the date to tmp, it gets added in ISO8601 format, so you need to use:
date {
match => ["tmp", "ISO8601"]
target => "#timestamp"
}