// структура самой записи в дневнике
type Expense struct {
  Date time.Date
  Sum float32
  Comment string
}
// Сам дневник
type Diary struct {
  Entries *list.List
}

type DiarySaveLoad interface {
  Save(diary *expenses.Diary)
  Load() *expenses.Diary
}

package io
import (
  "expenses/expenses"
  "fmt"
  "os"
)
type FileSystemDiarySaveLoad struct {
  Path string
}
func (f FileSystemDiarySaveLoad) Save(d *expenses.Diary) {
  file, err := os.Create(f.Path)
  if err != nil {
    panic(err)
  }
  for e := d.Entries.Front(); e != nil; e = e.Next() {
    buf := fmt.Sprintln(e.Value.(expenses.Expense).Date.Format(time.RFC822))
    buf += fmt.Sprintln(e.Value.(expenses.Expense).Sum)
    buf += fmt.Sprintln(e.Value.(expenses.Expense).Comment)
    if e.Next() != nil {
      buf += "\n"
    }
    _, err := file.WriteString(buf)
    if err != nil {
      panic(err)
    }
  }
  err = file.Close()
}

func (f FileSystemDiarySaveLoad) Load() *expenses.Diary {
  file, err := os.Open(f.Path)
  if err != nil {
    panic(err)
  }
  scanner := bufio.NewScanner(file)
  entries := new(list.List)
  var entry *expenses.Expense
  for scanner.Scan() {
    entry = new(expenses.Expense)
    entry.Date, err = time.Parse(time.RFC822, scanner.Text())
    if err != nil {
      panic(err)
    }
    scanner.Scan()
    buf, err2 := strconv.ParseFloat(scanner.Text(), 32)
    if err2 != nil {
      panic(err2)
    }
    entry.Sum = float32(buf)
    scanner.Scan()
    entry.Comment = scanner.Text()
    entries.PushBack(*entry)
    entry = nil
    scanner.Scan() // empty line
  }
  d := new(expenses.Diary)
  d.Entries = entries
  return d
}

package io
import (
  "container/list"
  "expenses/expenses"
  "math/rand"
  "testing"
  "time"
)
func TestConsistentSaveLoad(t *testing.T) {
  path := "./test.diary"
  d := getSampleDiary()
  saver := new(FileSystemDiarySaveLoad)
  saver.Path = path
  saver.Save(d)
  loader := new(FileSystemDiarySaveLoad)
  loader.Path = path
  d2 := loader.Load()
  var e, e2 *list.Element
  var i int
  for e, e2, i = d.Entries.Front(), d2.Entries.Front(), 0; e != nil && e2 != nil; e, e2, i = e.Next(), e2.Next(), i+1 {
    _e := e.Value.(expenses.Expense)
    _e2 := e2.Value.(expenses.Expense)
    if _e.Date != _e2.Date {
      t.Errorf("Data mismatch for entry %d for the 'Date' field: expected %s, got %s", i, _e.Date.String(), _e2.Date.String())
    }
    // аналогично проверяются остальные поля в Expense ...
  }
  if e == nil && e2 != nil {
    t.Error("Loaded diary is longer than initial")
  } else if e != nil && e2 == nil {
    t.Error("Loaded diary is shorter than initial")
  }
}
func getSampleDiary() *expenses.Diary {
  testList := new(list.List)
  var expense expenses.Expense
  expense = expenses.Expense{
    Date:    time.Now(),
    Sum:     rand.Float32() * 100,
    Comment: "First expense",
  }
  testList.PushBack(expense)
  // аналогично добавляются еще записи
  // ...
  d := new(expenses.Diary)
  d.Entries = testList
  return d
}

Data mismatch for entry 0 for the 'Date' field: expected 2020-09-14 04:16:20.1929829 +0300 MSK m=+0.003904501, got 2020-09-14 04:16:00 +0300 MSK

func Create(date time.Time, sum float32, comment string) Expense {
  return Expense{Date: date.Truncate(time.Second), Sum: sum, Comment: comment}
}