## TL;DR

Most policies pay a reduced rate (commonly 75 percent) for the first and last travel days. Check your company's partial-day rule, confirm the trip's departure and return times are accurate, and recalculate; the usual error is full days counted where partial days apply.

## Error

```text
per diem calculation wrong for partial days
```

## Steps

1. Look up your company's per diem policy for partial days (first/last day percentage). Expected: you know the exact rule, for example 75 percent.
2. Verify the trip's departure and return times in the expense tool. Expected: the timestamps reflect actual travel, not midnight defaults.
3. Recompute by hand: full days at 100 percent plus first and last days at the partial rate. Expected: a number you can compare against the tool.
4. Correct the timestamps if they are wrong and trigger a recalculation. Expected: the tool's total now matches your hand computation.
5. If the tool still disagrees, check whether it counts travel days by calendar date or by 24-hour periods. Expected: you find the counting convention causing the gap.

## When to use

- The per diem total is off by roughly one daily rate on a multi-day trip.
- Travel started or ended mid-day.

## When not to use

- No per diem calculated at all (check timestamps and rate configuration).
- The daily rate itself is wrong (ask the admin to fix the rate table).

## Tool compatibility

- Rydoo, Concur, Expensify per diem modules; policy rules per company.

## Variant phrasings

### Partial day per diem incorrect

Same problem, shorter wording.

### First and last day per diem wrong

Names the affected days directly.

## Why it happens

Per diem tools apply the partial-day rule to the trip's boundary days. Wrong timestamps (midnight defaults) or a misunderstood rule (full day vs 75 percent) shift the total by a large, noticeable amount.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_vn26RiU1_e1oQUE2lhZkMw