Two options, one is very quick and the other uses an array formula, which could be slow with decades worth of data.

The formula option: Array formula (confirmed using CTRL+SHIFT+ENTER, not just ENTER):

=AVERAGE(IF(INT($A$2:$A$1000)=F2,$B$2:$B$1000))

Change the reference to row 1000 to your last used row.

Other Option: Pivot Table

Select your data in columns A:B and click Insert > PivotTable. Drag the DateTime field to the Row Labels area. Drag the SG field to the Values area. Right-click on any of the DateTime entries and choose Group... and select Days so it is highlighted (you may need to de-select any other options). Right-click in the Values area of the pivot table and choose Summarize Data By.. > Average.

That should do the trick.