Stats heatmap counts a day active only if a session STARTED that day — days spent entirely in resumed sessions show 0 and break the streak

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

The usage stats screen (heatmap / current streak) appears to bucket
activity by the session's START date rather than by each message's own
timestamp.
Repro: work for a full day entirely inside a long-running session that
was started on an earlier day (resume across midnight, no new session
that day). The day shows 0 activity in the heatmap even though the
transcript contains real user/assistant messages timestamped that day,
and the current-streak counter breaks at it.
Concrete case: a day with 4 sessions / 428 message-lines (verified in
~/.claude/projects/**/*.jsonl, timestamps 06:49–17:25 UTC) rendered as
"0" because zero sessions had their first message on that date — all
activity lived in sessions started 5 days earlier.
Expected: a day is "active" if any message timestamp falls on it,
regardless of when the containing session started.
Env: claude-code 2.1.165, macOS.

What Should Happen?

A day in the usage stats should count as "active" if any message in any
transcript has a timestamp on that calendar day — regardless of when the
session that contains it was started.

Activity should be attributed per-message-timestamp, not per-session-
start date. A day worked entirely inside a long-running session that was
resumed from an earlier day should still light up in the heatmap and keep
the current streak intact.

Error Messages/Logs

A day in the usage stats should be counted as "active" if any message in
any transcript has a timestamp on that calendar day — regardless of when
the containing session was started.

Currently the heatmap / streak counter appears to bucket activity by the
session's START date, so a day spent entirely inside a long-running
session that was resumed from an earlier day shows 0 activity and breaks
the current-streak counter, even though that day's transcript contains
real user/assistant messages.

Expected: activity is attributed per-message-timestamp, not per-session-
start. Days worked entirely within resumed sessions should light up in
the heatmap and keep the streak intact.

Steps to Reproduce

1. Start a Claude Code session on day A (e.g. May 26) and do some work.
2. Keep that same session running and resume it over the following days

WITHOUT starting any new session — i.e. on day B (e.g. May 31) do all
your work inside that already-open session, across the midnight
boundary. (No new session has its first message on day B.)

1. Confirm real activity exists on day B: the transcript files in

~/.claude/projects/**/*.jsonl contain user/assistant messages with
timestamps on day B (in my case 4 sessions, 428 message-lines,
06:49–17:25 UTC).

1. Open the usage stats / heatmap screen.

Result: day B shows 0 activity in the heatmap and the current-streak
counter breaks at day B — even though day B has real messages. Every
neighbouring day (which had at least one session START that day) is
counted correctly. The only distinguishing factor of day B is that zero
sessions had their FIRST message on it.

Env: claude-code 2.1.165, macOS.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

claude-code 2.1.165

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_