seems the code is missing with the question, in case if you find trouble adding the code in question, add here in comment section

<grid string="Timesheets" adjustment="object" adjust_name="adjust_grid" create="true" edit="true">
<field name="validated" type="readonly" modifiers="{'readonly': true}"/>
<field name="employee_id" type="row" section="1" on_change="1" can_create="true" can_write="true" modifiers="{}"/>
<field name="project_id" type="row" on_change="1" can_create="true" can_write="true" modifiers="{}"/>
<field name="date" type="col" on_change="1" modifiers="{'required': true}">
<range name="week" string="Week" span="week" step="day"/>
<range name="month" string="Month" span="month" step="day"/>
</field>
<field name="unit_amount" type="measure" widget="timesheet_uom" string="Heures" on_change="1" modifiers="{}"/>
</grid>