From the web site of the Association of Cricket Statisticians and Historians.
Frequently Asked Questions.
When does a match begin?
Once the toss has been made, a match has officially started – so even if it is rained off without a ball having been bowled, it still counts as an ‘appearance’ for the 22 players involved.
When does an innings begin?
Each of the teams’ innings start upon the call of ‘Play’ by the umpire at the start of the innings. So if – as happened in the Third Test between England and Pakistan in August 2020 – two batsmen enter the playing area in the expectation of starting the innings, but play is then called off for the day without an umpire calling ‘Play’, the team is permitted to start its innings on the following day with different opening batsmen if it so wishes. Batsmen’s innings are timed from the moment when they cross the boundary into the playing area to start their personal innings. This does not, however, apply to the two opening batsmen, whose innings are timed from the call of ‘Play’ at the start of the team’s innings