--back <clock time> fails silently when time is on the previous calendar day

When using doing done past midnight with a --back time that refers to the previous calendar day, the command fails with an unhelpful error:

error: can't convert NilClass into an exact number

Steps to reproduce

1. It is 12:42am on April 25
2. Run: doing done "some entry" --back 2:30pm --section Home
3. Result: error: can't convert NilClass into an exact number

Expected behavior

--back 2:30pm at 12:42am should resolve to 2:30pm on April 24 — the most recent past occurrence of that clock time. Passing a time that is in the future on the current calendar day unambiguously means "yesterday at that time."

Workaround

Using a duration instead of a clock time works correctly:

doing done "some entry" --back 612m --section Home

Additional context

The error message gives no hint that the issue is a day-boundary ambiguity. "Can't convert NilClass into an exact number" reads like an internal null reference rather than a user-fixable input problem. Even if the "assume previous day" behavior isn't implemented, a clearer error like "time is in the future — did you mean yesterday?" would help.

Start with the doing done command and reproduce the failure using --back 2:30pm just after midnight. Trace how the clock time is resolved across the calendar-day boundary, then verify that it selects the previous day or reports a clear actionable error instead of the NilClass conversion message.