improve perf 3-5x by avoiding re-instantiating date formatter

[pelhage]

Description

Querying workouts was taking an unexpectedly long time, so did a perf trace, saw that this code was the culprit:

Modified the code and re-ran my project and saw huge savings.

TBH not sure if this needs more extensive testing (are there unit tests?), but feedback is welcome.

BEFORE:

AFTER:

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have checked my code and corrected any misspellings