Skio Analytics counts days in your store's timezone, which is the timezone set in your Shopify admin. If you change your store's timezone, past days are recalculated using the new timezone.
A few reports count days in UTC (Coordinated Universal Time) instead:
Loyalty Analytics: Tiers: Tier placements are counted in UTC. Tier order and revenue numbers still use your store's timezone.
Surprise & Delight: Rewards triggered are counted in UTC.
Swap recommendations: Swap stats are counted in UTC.
Performance: The older added MRR and cancelled MRR totals are counted in UTC.
For these reports, an event that happens close to midnight in your timezone can be counted on the day before or after. This only affects daily numbers. Totals over a week or a month stay the same.