The twenty-four hour clock: one number for every hour
The twenty-four hour clock counts the hours of the day from 00:00 to 23:59, so every hour has its own number and there is no am or pm.
In the Netherlands, as in most of Europe, times are written on the twenty-four hour clock. Morning hours look familiar: 09:00 is nine in the morning. After midday the count goes on: 13:00, 14:00, up to 23:00, which is eleven at night.
Changing between the two
For an hour after midday, take away twelve to get the twelve hour clock: 20:00 is 8 pm. From pm back to the twenty-four hour clock, add twelve: 11 pm is 23:00.
The midnight trap
At 00:00 the date changes. So 00:30 on Sunday is the night between Saturday and Sunday, not late on Sunday. Name both days for any hour after midnight.
Why this site uses it
Because it cannot be misread. Nine can be morning or evening. 21:00 can only be evening. Every hour in a reply is written this way, and in Amsterdam time. The guide to hours and dates has a full table of evening and night hours.