Calendar has disappeared
Please check the calendar exists in the Calendar app.
If it is displayed in the Calendar app, it will be displayed Timesheet app.
When you were using the iCloud Calendar,
Confirm that you've turned on iCloud Calendar by tapping Settings > iCloud on your iPhone, then check that Calendars is turned on.
Please refer also iCloud: Troubleshooting iCloud Calendar
Data of a few months ago has disappeared
If you've been using the calendar of iCloud, you may be able to recover by changing the settings.
  1. Open your iPhone's "Settings" > "Mail, Contacts, Calendar"
  2. You'll change to "All Events", the value of "CALENDARS" > "Sync".
  3. Open the "Calendar" app of iPhone, and wait for data to be synchronized.
When Timezone changed, Data of the day is no longer displayed
It might not work well if Time Zone Support is turned on.
Configuration of Timezone support is located in Settings > Mail,Contacts, Calendars.
If your timezone changes, Please turn off this.
Is there mac app?
But, using such as Mac standard calendar app, it is possible to register on Mac.
You can enter "tag" a comma-separated in the first line of the Notes field.
You can enter "memo" in the second and subsequent lines of the Notes field.
How do I back up?
Please use the backup function of calendar.

If you are using the iCloud calendar, data on iCloud has been saved.
Therefore, if you delete the app, data is stored.
To further perform a backup, it is possible to use the standard Mac "Calendar" app, it can be a export as file.
Please refer How to back up iCal calendar data

If you are using Google Calendar, please click here to reference. Export your calendar
How do I sync between iPhone and iPad?
Please try to synchronize your calendars.

If you are using a calendar of iCloud on Timesheet, Settings > iCloud > Calendars turn ON.
When the same settings of iPhone and iPad, your calendars will be synchronized.