Fix previous_summary from summary timer

[ghost]

Description

Retrieval of the previous summary date from another date didn't work as expected.
This PR fixes this.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[ghost]

Current dependencies on this PR:

• Fix beacon live transaction chain loading #360
• Write beacon slot to summary chain #358