If you wish to set all of your employee's leave start year to start on a set date, for example the 01 April every year, you will need to change the leave category to 'Automatically Accrues' and select 'Based on the employee's leave year' from the drop down menu and press save. To do this go to Payroll Settings > Pay Run Settings > Leave Categories and select the leave category you wish to alter. See screen shot below.
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Once you have done this you need to go to each employee's leave allowances and ensure that their leave year starts on the 01 April and that they have their leave set to 'Based on the employee's leave year' as pictured below.