https://www.polygon.com/2018/3/2/17071158/nintendo-switch-playtime-hours-deleted-removed-activity-log

Nintendo users are finding that their logged playtime data from one year ago, when the console launched, is gone. It appears that the system is resetting the amount of time for which games have been played since on or before March 2, 2017 — suggesting that the Switch only tracks the month and day players first started up a game, not the year.

This is best explained with an illustrative example. Polygon received a pre-release Nintendo Switch review unit on Feb. 24, 2017. We began playing The Legend of Zelda: Breath of the Wild that same day. Although we’ve poured hundreds of hours into the game since then (how else do you think we could have made our guides?), that same Nintendo Switch now says we only started playing Breath of the Wild six days ago. It appears that sometime around when the console turned 1 year old, it erased 365 days’ worth of activity from the Zelda playtime log.