fix: update sleep events if their duration increases

[MatthewMazaika]

the two timestamps being compared were strings, not date objects - they were also in different timezone formats

event["time"]                 => '2024-05-19 23:30:16-05:00'
last_upload["created_at"]     => '2024-05-20T04:30:16.000Z'

after casting the strings to date objects and then doing the comparison, the sleep events get updated properly

i believe this resolves #18