ExtJS4 TimeField increment

I just want to have 2 (08:00 and 13:00) values in a TimeField :开发者_StackOverflow社区

xtype: 'timefield',
name: 'operation[starthour]',
editable: false,
minValue: '08:00',
maxValue: '13:00',
value: '08:00',
increment: 300

It doesn't work at all. With this code I just have 10:00 value in the combobox.

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The docs say:

The number of minutes between each time value in the list.

An increment of 300 means 5 hours.