How can I work out time difference between git tag and the original commit

Question

I am looking to work out the time difference between when a commit is made and when that commit was tagged.

This is what I have so far however the committerdate which I think this is the correct field is null.

git for-each-ref --format='%(taggerdate) : %(committerdate) :  %(refname)' --sort=-taggerdate --count=10 refs/tags

At a guess I am looking only at tags, though I cannot get it right. In an ideal world it would show the difference between the times however I can do this afterwards if it is not possible.

Answer 1

The git for-each-ref documentation is kind of misleading:

For commit and tag objects, the special creatordate and creator fields will correspond to the appropriate date or name-email-date tuple from the committer or tagger fields depending on the object type. These are intended for working on a mix of annotated and lightweight tags.

This does work exactly as documented: %(creatordate) gets the date from the tagger field of these annotated tags. However, it seems to imply that for an annotated tag, Git might inspect the corresponding commit: if not, how would it ever get the committer?

In fact, for-each-ref never looks directly at an annotated tag's target object. The mention of the committer field is specific to lightweight tags, which point to commits,¹ rather than annotated tag objects. Fortunately, there's a format directive saying to "look indirectly", as it were: simply prefix the directive with *.²

Hence the answer is:

git for-each-ref \
    --format='%(taggerdate) : %(*committerdate) :  %(refname)' \
    --sort=-taggerdate --count=10 refs/tags

---

¹Lightweight tags can point directly to trees or blobs; it's not immediately clear what happens in this case. Probably the expansion is empty.

²Note that this only indirects once, so if the target of the annotated tag is another tag (or is a tree or blob), there will be no committer date field. To really make this reliable, you probably should use a bit of shell script and resolve the tag name with the ^{} or ^{commit} suffix. (The difference between these two is that ^{commit} will fail if the ultimate target object is a tree or blob, while ^{} will succeed, finding that tree or blob object.)