Fixed backup filename date and time ...

- fixed derp where i had set the date string to be read on app startup, this caused two problems...

1. if the app was open for a time and then a backup was made the filename date and time would be
set to when the app was opened and not when the actual backup was taken
2. also if more than one backup was made without restarting the app, all relevant backups would be
named the same as the date and time string was only requested on app startup.

This commit fixes both of the above problems