The above code is working with me in Odoo 15.0. Make sure to use self before the field name.

What Odoo version you are using?
What the result you are getting?

Could you please post your code?

@Waleed Mohsen brother check:
from odoo import fields, api, models
from odoo.exceptions import ValidationError
from datetime import datetime

class Regular(models.Model):
_name = 'attendance.regular'
_rec_name = 'employee_id'
_description = 'Approval Request'
_inherit = ['mail.thread', 'mail.activity.mixin']
from_date = fields.Datetime(string='Start', required=True, compute="_compute_time")
to_date = fields.Datetime(string='End')
def _compute_time(self):
for rec in self:
if rec.from_date:
rec.datetime.strftime(self.from_date, "%H:%M:%S")

For XML view :
<field name="from_date"/>

from odoo import fields, api, models
from odoo.exceptions import ValidationError
from datetime import datetime

class Regular(models.Model):
_name = 'attendance.regular'
_rec_name = 'employee_id'
_description = 'Approval Request'
_inherit = ['mail.thread', 'mail.activity.mixin']

from_date = fields.Datetime(string='Start', required=True)
from_time = fields.Char(compute="_compute_time")
to_date = fields.Datetime(string='End')

def _compute_time(self):
for rec in self:
if rec.from_date:
rec.from_time = datetime.strftime(rec.from_date, "%H:%M:%S")