Start date in irregular series is not same with date which is shown on mouse-over tip.
Also, you can check this on HC demo page for Winter serie start date: http://www.highcharts.com/demo/spline-irregular-time

How can I fix this, so both date point to regular one.
TY
There is no error. XAxis labels is calculated for only to fit the screen. There may be no point on label. For this example, time is fragmented per-14-days. If time is too long, may be fragmented more than 14, OR less than 14.