[FIX] use contract date rather than invoice date

Invoice date might not the date of the invoiced period, because invoice are not validated and sent on the first day of the period.

contract_timesheet_monitoring/models/account_analytic_account.py

@@ -8,8 +8,6 @@ def get_time_spent_for_period(self, start_date, end_date=None):
         analytic_account_lines = self.line_ids
         timesheets = analytic_account_lines.filtered(
             # keep only timesheets
-            # ensure the uom is the same as the one configure for the project
-            # timesheets (hours or day)
             lambda x: (x.product_uom_id.measure_type == "time")
         )
         if timesheets:

contract_timesheet_monitoring/models/contract_line.py

@@ -1,4 +1,5 @@
 from odoo import api, fields, models
+from odoo.exceptions import UserError
 
 
 class ContractLine(models.Model):
@@ -14,18 +15,20 @@ class ContractLine(models.Model):
     )
     date_of_last_invoice = fields.Date(compute="_get_date_of_last_invoice")
 
-    def _get_date_of_last_invoice(self):
-        invoices = self.contract_id._get_related_invoices().filtered(lambda x: (x.state != "draft"))
-        if any(invoices.mapped("date_invoice")):
-            return max(invoices.mapped("date_invoice"))
-        else: 
-            return False
+    def _get_period_start_date(self):
+        if self.recurring_invoicing_type == 'post-paid':
+            start_date = self.recurring_next_dat
+        else:
+            start_date = self.recurring_next_date - self.get_relative_delta(
+                            self.recurring_rule_type, self.recurring_interval
+                        )
+        return start_date
 
     @api.depends("analytic_account_id.line_ids")
     def _compute_time_spent(self):
         for line in self:
             if line.analytic_account_id:
-                period_start_date = line._get_date_of_last_invoice()
+                period_start_date = line._get_period_start_date()
                 line.time_spent = line.analytic_account_id.get_time_spent_for_period(
                     period_start_date
                 )