To be more specific, the contribution limit is actually divided in 12 for each month you're an eligible individual (i.e. covered only by HDHP insurance and no other).
If you're covered by an HDHP for all of 2019, you get to make the full $3,500 contribution. If you switch to some other coverage at the end of April, you only get to contribute $1,166.66 ($3,500 / 12 * 4).
Furthermore there's a "last-month rule" that allows you to make a full-year contribution if you have HDHP coverage for December and maintain that coverage for the entire following year. So if you had switched to HDHP coverage starting on or before December 1 of last year, you would be able to make a full $3,450 contribution for 2018 provided you intend to continue with an HDHP for all of 2019. However if your coverage switched on January 1 instead, the 2018 contributions are not available to you.