Is the Swiss PSVA for a single day valid for a given day or for 24 hours?

The calendar day is decisive. 

For example: From payment, the PSVA is valid until 11.59pm the same day. If the form is paid after 7pm, it is valid until 11.59pm the following day.

Related articles:

Do I need the Swiss PSVA on ordinary roads?

Overstaying paid Swiss PSVA

Can I obtain a refund for the PSVA when leaving Switzerland earlier?

Can I extend the Swiss PSVA?

No proof of payment for the Swiss PSVA