The only think we can be certain of is that the Summer Solstice occurs when the mid summer sun shines through a trillithon at Stonehenge and strikes a certain point. From there we can work outwards.
The logical thing to do is to precisely model Stonehenge to the last micron in space. That will take a bit of work involving the various sea levels and so on. So on will include the thermal expansion of granite and the traffic density on the A303 and whether the Solstice is a bank holiday.
Oh bollocks ... mass. That standard kilo thing - is it sorted out yet? Those cars and lorries are going to need constant observation - we'll need a sort of dynamic weigh bridge that works at 60mph. If we slap it in the road just after (going west) the speed cameras should keep the measurements within parameters. If we apply now, we should be able to get Highways to change the middle of the road markings from double dashed to a double solid line and then we can simplify a few variables.
... more daft stuff ...
Right, we've got this. We now have a standard place and point in time to define place and time from.
No we don't and we never will. There is no absolute when it comes to time, place or mass. What we do have is requirements for standards and a point to measure from. Those points to measure from have differing requirements, depending on who who you are and what you are doing.
I suggest we treat time as we do sea level, with a few special versions that people can use without having to worry about silliness.
Provided I can work out when to plant my wheat crop and read log files with sub micro second precision for correlation, I'll be happy. My launches to the moon will need a little more funkiness ...